

r 


document, all classification THIS 
masking s must BE~CANCElT,W- 


/W33. 






SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


•declassified 

By authority Secretary of 

OCT 71960 

Defense memo 2 August 1960 
LIBRARY OF CONGRESS 


This document contains information affecting the national defense of the 
United States within the meaning of the Espionage- Act, 50 U. S. C., 31 and 
32, as amended. Its transmission or the revelation of its contents in any 
manner to an unauthorized person is prohibited by law. 

This volume is classified CONFIDENTIAL in accordance with security 
regulations of the War and Navy Departments because certain chapters 
contain material which was CONFIDENTIAL at the date of printing. 
Other chapters may have had a lower classification or none. The reader is 
advised to consult the War and Navy agencies listed on the reverse of this 
page for the current classification of any material. 


CONFIDENTIAL 






Manuscript and illustrations for this volume were prepared for 
publication by the Summary Reports Group of the Columbia 
University Division of War Research under contract OEMsr-1131 
with the Office of Scientific Research and Development. This vol¬ 
ume was printed and bound by the Columbia University Press. 

Distribution of the Summary Technical Report of NDRC has 
been made by the War and Navy Departments. Inquiries concern¬ 
ing the availability and distribution of the Summary Technical 
Report volumes and microfilmed and other reference material 
should be addressed to the War Department Library, Room 
1A-522, The Pentagon, Washington 25, D. C., or to the Office of 
Naval Research, Navy Department, Attention: Reports and 
Documents Section, Washington 25, D. C. 


Copy No. 

2?9 


This volume, like the seventy others of the Summary Technical 
Report of NDRC, has been written, edited, and printed under 
great pressure. Inevitably there are errors which have slipped 
past Division readers and proofreaders. There may be errors of 
fact not known at time of printing. The author has not been able 
to follow through his writing to the final page proof. 


Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports and 
sent to recipients of the volume. Your help will make this book 
more useful to other readers and will be of great value in prepar¬ 
ing any revisions. 


n 


CONFIDENTIAL 


SUMMARY TECHNICAL REPORT OF DIVISION 16, NDRC 


VOLUME 2 


VISIBILITY STUDIES AND SOME 
APPLICATIONS IN THE FIELD 
OF CAMOUFLAGE 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 16 

GEORGE R. HARRISON, CHIEF 


- PECLASHTPTcip 

By authority Secretary 0 f 

WASHINGTON, D. C., 1946 

°CT 7I960 

Defense m emo 2 A ugust I960 

LIBRARY of congress 


COM I DIM i \r. 







NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative 1 

Frank B. Jewett Navy Representative 2 

Karl T. Compton Commissioner of Patents 3 

Irvin Stewart, Executive Secretary 


1 Army representatives in order of service: 


Maj. Gen. G. V. Strong 
Maj. Gen. R. C. Moore 
Maj. Gen. C. C. Williams 
Brig. Gen. W. A. Wood, Jr. 

Col. E. A. 


Col. L. A. Denson 
Col. P. R. Faymonrille 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
Routheau 


2 Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 

Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 

Commodore H. A. Schade 
3 Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities of 
warfare, together with contract facilities for carrying out 
these projects and programs, and (2) to administer the tech¬ 
nical and scientific work of the contracts. More specifically, 
NDRC functioned by initiating research projects on re¬ 
quests from the Army or the Navy, or on requests from an 
allied government transmitted through the Liaison Office 
of OSRD, or on its own considered initiative as a result of 
the experience of*its members. Proposals prepared by the 
Division, Panel, or Committee for research contracts for 
performance of the work involved in such projects were 
first reviewed by NDRC, and if approved, recommended to 
the Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of the 
contract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and administra¬ 
tion of patent matters were handled by the Executive Sec¬ 
retary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC* members. 
These were: 

Division A—Armor and Ordnance 

.Division B—Bombs, Fuels, Gases, & Chemical Problems 
Division C—Communication and Transportation 
Division D—Detection, Controls, and Instruments 
Division E—Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three ad¬ 
ministrative divisions, panels, or committees were created, 
each with a chief selected on the basis of his outstanding 
work in the particular field. The NDRC members then be¬ 
came a reviewing and advisory group to the Director of 
OSRD. The final organization was as follows: 

Division 1—Ballistic Research 

Division 2—Effects of Impact and Explosion 

Division 3—Rocket Ordnance 

Division 4—Ordnance Accessories 

Division 5—New Missiles 

Division 6—Sub-Surface Warfare 

Division 7—Fire Control 

Division 8—Explosives 

Division 9—Chemistry 

Division 10—Absorbents and Aerosols 

Division 11—Chemical Engineering 

Division 12—Transportation 

Division 13—Electrical Communication 

Division 14—Radar 

Division 15—Radio Coordination 

Division 16—Optics and Camouflage 

Division 17—Physics 

Division 18—War Metallurgy 

Division 19—Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


IV 


CONFIDENTIAL 



NDRC FOREWORD 


A s events of the years preceding 1940 revealed 
- more and more clearly the seriousness of the 
world situation, many scientists in this country 
came to realize the need of organizing scientific re¬ 
search for service in a national emergency. Recom¬ 
mendations which they made to the White House 
were given careful and sympathetic attention, and 
as a result the National Defense Research Commit¬ 
tee [NDRC] was formed by Executive Order of the 
President in the summer of 1940. The members of 
NDRC, appointed by the President, were instructed 
to supplement the work of the Army and the Navy 
in the development of the instrumentalities of war. 
A year later, upon the establishment of the Office 
of Scientific Research and Development [OSRD], 
NDRC became one of its units. 

The Summary Technical Report of NDRC is a 
conscientious effort on the part of NDRC to sum¬ 
marize and evaluate its work and to present it in a 
useful and permanent form. It comprises some 
seventy volumes broken into groups corresponding 
to the NDRC Divisions, Panels, and Committees. 

The Summary Technical Report of each Division, 
Panel, or Committee is an integral survey of the 
work of that group. The first volume of each group’s 
report contains a summary of the report, stating the 
problems presented and the philosophy of attacking 
them, and summarizing the results of the research, 
development, and training activities undertaken. 
Some volumes may be “state of the art” treatises 
covering subjects to which various research groups 
have contributed information. Others may contain 
descriptions of devices developed in the laboratories. 
A master index of all these divisional, panel, and 
committee reports which together constitute the 
Summary Technical Report of NDRC is contained 
in a separate volume, which also includes the index 
of a microfilm record of pertinent technical labora¬ 
tory reports and reference material. 

Some of the NDRC-sponsored researches which 
had been declassified by the end of 1945 were of 
sufficient popular interest that it was found desir¬ 
able to report them in the form of monographs, such 
as the series on radar by Division 14 and the mono¬ 


graph on sampling inspection by the Applied Mathe¬ 
matics Panel. Since the material treated in them 
is not duplicated in the Summary Technical Report 
of NDRC, the monographs are an important part 
of the story of these aspects of NDRC research. 

In contrast to the information on radar, which is 
of widespread interest and much of which is released 
to the public, the research on subsurface warfare is 
largely classified and is of general interest to a 
more restricted group. As a consequence, the report 
of Division 6 is found almost entirely in its Sum¬ 
mary Technical Report which runs to over twenty 
volumes. The extent of the work of a division can¬ 
not therefore be judged solely by the number of 
volumes devoted to it in the Summary Technical 
Report of NDRC; account must be taken of the 
monographs and available reports published else¬ 
where. 

Division 16 carried out a broad program in the 
fields of light and optics. Among the studies under¬ 
taken were a number involving the principles and 
techniques of camouflage, and perhaps the outstand¬ 
ing success achieved in this field was the develop¬ 
ment of the “black widow” finish for night-flying 
aircraft. Significant improvements were made in 
aerial mapping and photography. Devices depend¬ 
ing on the use of infrared light were developed for 
the detection of enemy craft, the recognition of 
friendly ones, and for intercommunication by voice 
and code. The sniperscope, using image-forming 
infrared rays, was a spectacular weapon which en¬ 
abled our troops to fire accurately on an enemy 100 
yards away in utter darkness. 

The Division 16 Summary Technical Report, pre¬ 
pared under the direction of the Division Chief, 
George R. Harrison, describes the technical achieve¬ 
ments of the Division personnel and its contractors, 
and is a record of their skill, integrity and loyal 
cooperation. To all of them, we extend our grateful 
praise. 

Vannevar Bush, Director 
Office of Scientific Research and Development 
J. B. Conant, Chairman 
National Defense Research Committee 


CONFIDENTIAL 


v 





FOREWORD 


A t the time of its formation late in 1942, Divi- 
- sion 16, the Optics Division of NDRC, was 
assigned both the general task of stimulating and 
supervising OSRD research in optics and the imme¬ 
diate problem of overseeing a large number of con¬ 
tracts which had previously been initiated by the 
Instruments Section. Inasmuch as the new Division 
consisted to a large extent of personnel associated 
with the Instruments Section during 1940 and 1941, 
the reorganization involved few important changes. 

The present Summary Technical Report describes 
the accomplishments of both Division 16 and Sec¬ 
tion D-3, and covers the principal developments in 
optics made in America during World War II. This 
report should be considered as intermediate in char¬ 
acter between the detailed contractors’ reports of 
Division 16, to which reference is frequently made 
herein which are complete scientific reports of the 
investigations carried on, and the historical volume 
entitled Optics and Applied Physics in World War 
II, which presents in less technical form the accom¬ 
plishments of the Division and its contractors, and 
assigns credit to those who took part. 

The contents of the present volume demonstrate 
impressively the great contribution made by the 
optical industry of America and the university opti¬ 
cal laboratories to the war effort. While less glamor¬ 
ous than some of the newer fields brought into 
existence during the war, optics nevertheless made 
significant contributions which were by no means 
confined to mere extension or application of optical 
methods or apparatus previously in use. The stress 
of the emergency produced many new optical de¬ 


velopments, and the genesis of a large proportion 
of these will be found recorded in the following 
pages. 

The science of optics and the optical industry 
have both benefited greatly by the intensive re¬ 
search which took place during the war. Many of 
the new devices developed under emergency condi¬ 
tions have contributed and will contribute more to 
our fundamental understanding of optics, and many 
of them will have peacetime applications. New lines 
along which optical research should be directed 
have been made apparent. In particular, the infra¬ 
red field has benefited greatly, and the art of in¬ 
frared phosphor development and utilization has 
been elevated to an entirely new level. 

Consideration of the developments in optics, as 
in other fields, emphasizes that, once adequate im¬ 
mediate defense has been insured, more important 
than having weapons for a possible future war is 
having available a large body of trained personnel 
who can step into any breach that occurs and be 
available to produce the new devices that may be 
needed. 

The Optics Division of NDRC is especially in¬ 
debted to the chiefs and members of its Sections, 
whose names are listed at the end of this volume. 
They have provided the essential leadership, com¬ 
bined with scientific knowledge, without which the 
work of the Division could not have been planned 
or completed. 

George R. Harrison 
Chief, Division 16 


vii 


CONFIDENTIAL 







PREFACE 


T his book summarizes the principal activities of 
the Camouflage Section of NDRC (Section 10.3) 
during World War II as they appear in retrospect 
more than a year after the Section ceased to exist 
as a body and more than eight months after the 
close of hostilities. Unlike most groups in the NDRC 
organization, the Camouflage Section was not pri¬ 
marily concerned with the development of the in¬ 
strumentalities of war but rather with techniques 
for their concealment. By studying the inherent 
limitations of human vision, and by making proper 
allowance for the effects of the atmosphere, the con¬ 
cealment aspects of camouflage have been reduced, 
in most cases, to an engineering procedure. The re¬ 
sults of these studies naturally have an important 
bearing on the solution of military and naval visi¬ 
bility problems of all kinds. 

Because the subject of camouflage has little peace¬ 
time interest, the Section was forced to create a 
large part of its research facilities. For the same 
reason, it seemed prudent to concentrate the efforts 
in a few central laboratories. A central laboratory 
for camouflage field studies was established at the 
Louis Comfort Tiffany Foundation, Oyster Bay, 
New York, and many of the Section’s activities were 
centered there. Because of the many special prob¬ 
lems that arise in connection with camouflage fin¬ 
ishes, another central laboratory was established at 
the Research Laboratories of the Interchemical 
Corporation, New York City. These two labora¬ 
tories, because of their propinquity, supplemented 
each other very effectively. For example, the Black 
Widow finish, which provides effective antisearch¬ 
light protection for aircraft, was developed at the 
Research Laboratories of the Interchemical Cor¬ 
poration, following suggestions by the Section Chief. 
This finish was supplied, in turn, to the Tiffany 
Foundation, where its value was demonstrated by 
field tests on a model scale. 

The activities of the Section’s contractors have 
been recorded in the customary contractor’s reports. 
Eleven of these reports have been published in uni¬ 
form format and binding, and each contains a fore¬ 
word by the Section which explains its relation to 
the war effort. The purpose of this Summary Tech¬ 
nical Report is to present the work of the con¬ 
tractors in abstract form and to supply enough 
coordinating material to make the results useful to 


the Armed Services. It has been divided into three 
parts: the first is a broad summary of the entire 
program; the second an interpretation by Section 
personnel of the researches on the general subject of 
visibility; and the third is an account of two proj¬ 
ects that seemed of sufficient importance to justify 
more than the usual summary, namely, the Yehudi 
project and the Black Widow project. 

The members of the Camouflage Section were 
Arthur C. Hardy, Chief, Massachusetts Institute of 
Technology; Edwin G. Boring, Harvard University; 
Herbert E. Ives, Bell Telephone Laboratories; 
Lloyd A. Jones, Eastman Kodak Company; and 
Frank C. Whitmore, The Pennsylvania State Col¬ 
lege. The Technical Aides were Seibert Q. Duntley, 
Massachusetts Institute of Technology; Arthur W. 
Kenney, E. I. du Pont de Nemours & Company, 
Inc.; and Ernest T. Larson, now with General Ani¬ 
line & Film Corporation. Consistent with the sev¬ 
eral aspects of camouflage studies, the personnel of 
the Section represented widely diversified interests. 
These interests include optical physics (Duntley, 
Hardy, Ives, Jones), psychology (Boring), chemis¬ 
try and chemical engineering (Kenney, Whitmore), 
meteorological optics (Duntley), photometry and 
spectrophotometry (Duntley, Hardy, Larson). 

The published history of OSRD can be consulted 
for a complete list of all the many contractors’ per¬ 
sonnel who contributed to the success of the work. 
However, mention should be made here of certain 
persons whose contributions proved to be especially 
important: Kenneth V. Thimann of Harvard Uni¬ 
versity developed a chlorophyll paint from plant 
materials; Betty T. Mesier of American Cyanamid 
Company developed a camouflage for water surfaces 
originally suggested by E. L. Kropa of the same 
company; Carl E. Foss of the Tiffany Foundation 
conducted the original experiments on Yehudi cam¬ 
ouflage and later J. W. Tumavicus of Pratt, Read & 
Company was in charge of applying this camouflage 
to Navy Glombs. John E. Tyler and Harry E. 
Rose of Interchemical Corporation developed an 
automatic photoelectric control mechanism for the 
Yehudi lamps; Edward C. Dench, Charles A. Wes¬ 
ley, and Clifton B. Kinne of the Interchemical 
Corporation built numerous special electronic in¬ 
struments; and David L. MacAdam of the Eastman 
Kodak Company designed and supervised the con- 


CONFIDENTIAL 


IX 


struction of the spectrogeograph. Later, MacAdam 
supervised an investigation of the visibility of col¬ 
ored targets, and was the author of several contrac¬ 
tors' reports as well as Chapter 3 of this volume. 
Willard P. Greenwood of the Tiffany Foundation 
operated the spectrogeograph on all of its flights and 
was in charge of the research program which cen¬ 
tered about that instrument. Meteorological corre¬ 
lations with the spectrogeograph experiments were 
provided by C. A. Elford of the U. S. Weather 
Bureau, who accompanied Greenwood on his flights 
during the spring of 1944. H. Richard Blackwell 
was in charge of the visibility research program at 
the Tiffany Foundation during the time when all of 
the data reported in this volume were secured. Dur¬ 
ing the earlier part of the visibility program at 
Tiffany, psychometric techniques were developed 
by Helen Peak and Helen M. Richardson. William 
F. Little of the Electrical Testing Laboratories 
served throughout the project as a consultant in 
photometry. The special apparatus required for the 
visibility research was devised and constructed by 
Carl E. Foss, William Kerbs, Schell Lewis, and 
Benjamin Pritchard. The nomographic visibility 
charts were prepared by Raymond D. Douglass of 
Massachusetts Institute of Technology, who served 
as the consultant in mathematics to the Tiffany 
Foundation. 

The chemical projects summarized in this volume 
were carried out by the staff of the Research Lab¬ 
oratories of the Interchemical Corporation, directed 
by Albert E. Gessler. Investigations involving pig¬ 
ments, enamels, and lacquers were supervised by 
Earl K. Fischer, Edmund N. Harvey, and P. A. 
Henry. Walter C. Granville was in charge of spec- 
trophotometric measurements and calculations. The 
synthesis and manufacture on a pilot plant scale of 
novel pigments were executed in the organic field by 


Sylvester A. Scully, and in the inorganic field by 
Charles A. Kumins. Problems concerned with the 
paint vehicles were handled by L. S. Ingle and C. J. 
Rolle. The efforts of all these groups were coordi¬ 
nated by David M. Gans on behalf of the Inter¬ 
chemical Corporation. 

The efforts of the Camouflage Section were aided 
by the liaison officers assigned to its projects by 
the Armed Forces and by numerous other officers 
who, although not officially designated as liaison 
officers, followed the work with keen interest and 
helped in many ways. Among the long list of liaison 
officers special mention should be made of the fol¬ 
lowing men who visited the laboratories on many 
occasions, accompanied field expeditions, and pro¬ 
vided invaluable assistance: Captain Charles Bit- 
tinger, BuShips; Commander Dayton R. E. Brown, 
BuShips; Lieutenant Commander David F. Leavitt, 
BuAer; Major Arthur W. Van Heuckeroth, Corps 
of Engineers; and Major F. L. Wurzburg, A.A.F. 
The Section gratefully acknowledges the courtesies 
extended by the staff Photo Technical Unit, 
AAFTAC, Orlando, Florida, and to Major John 
Larkin, Captain P. K. Rock, and Lieutenant S. T. 
Jennings for their services in flying the spectrogeo¬ 
graph. The adoption of the Black Widow antisearch¬ 
light camouflage by the United States and British 
Air Forces was due primarily to the efforts of Major 
Paul L. Hexter, A.A.F., who conducted the original 
flight tests at Eglin Field and subsequently intro¬ 
duced this camouflage measure in England and 
throughout the Pacific theater. The OSRD Office of 
Field Service cooperated with Major Hexter in 
bringing about the adoption of the Black Widow 
finish in all theaters of operation. 

Seibert Q. Duntley 
Technical Aide, Section 16.3 


x 


CONFIDENTIAL 


CONTENTS 


PART I 

SUMMARY OF THE ACTIVITIES OF THE 
NDRC CAMOUFLAGE SECTION 

CHAPTER PAGE 

1 Introduction and Summary ..... 3 

PART II 

VISIBILITY OF TARGETS 

2 The Screening of Targets by the Atmosphere . . 19 

3 Perceptual Capacity of the Human Observer . . 33 

4 The Visibility of Naval Targets .... 74 

5 Visibility from Aircraft ...... 138 

PART III 

AIRCRAFT CAMOUFLAGE 

6 Camouflage of Sea-Search Aircraft .... 225 

7 Antisearchlight Camouflage for Aircraft . . . 242 

Appendix A . . . . . . . . 251 

Glossary ........ 257 

Bibliography ........ 281 

OSRD Appointees ....... 264 

Contract Numbers ....... 267 

Service Project Numbers ...... 268 

Index ......... 269 


CONFIDENTIAL 









PART l 

SUMMARY OF THE ACTIVITIES OF THE 
NDRC CAMOUFLAGE SECTION 








Chapter 1 

INTRODUCTION AND SUMMARY 


11 INTRODUCTION 

O n December 10, 1941, the National Defense Re¬ 
search Committee [NDRC] appointed an ad hoc 
Committee on Camouflage to “review the status of 
camouflage developments, the research now under 
way, and to make recommendations to Dr. Vanne- 
var Bush in regard to extension of the present 
research.” The final report 1 of the ad hoc commit¬ 
tee begins with the following discussion of the 
definition of camouflage: 

The term camouflage came into use in France during 
World War I to describe certain defensive measures made 
necessary by the introduction of new offensive weapons, prin¬ 
cipally the airplane and the submarine. If interpreted 
broadly, deception may be regarded as a synonym for camou¬ 
flage. The deception may comprise concealment of the type 
exemplified by the protective coloration in the case of ani¬ 
mals, or it may merely create confusion with respect to the 
identity or velocity of the objective (especially a ship), a 
form of camouflage that is sometimes equally effective and 
less difficult of attainment. To include all the ramifications 
of the subject, camouflage must be understood to include the 
use of smoke screens, dummies, and other deceptive prac¬ 
tices. 

Although the detection or breaking of camouflage is a sep¬ 
arate profession when considered with respect to the methods 
employed, it is closely related in the sense that it defines the 
problem of the camoufleur, who undertakes to achieve a 
concealment that cannot be penetrated. It so happens that 
the principal methods of detection possess important peace¬ 
time applications that have fostered their continuous devel¬ 
opment. Camouflage, being essentially a wartime activity, 
has not received a corresponding amount of attention during 
the years of peace. As a consequence, advances in camou¬ 
flage techniques have not kept pace with improvements in 
the techniques of detection. Because this report is concerned 
primarily with the research and development aspects of 
camouflage, it must necessarily survey the methods of detec¬ 
tion which are now available or are in the process of devel¬ 
opment. 

In its most elementary form, camouflage undertakes to 
provide concealment against detection by the unaided human 
eye. Because of the widespread use of photography and the 
possible use of image-tubes and other visual aids, however, 
the requirements of successful concealment have become 
more stringent. By extension, camouflage has sometimes come 
to mean concealment against any method of detection. Thus, 
the firing of several guns simultaneously may provide con¬ 
cealment against sound ranging; and the jamming of radio 
signals may hide an objective from radio detection. For the 
purpose of this report, camouflage is understood to consist of 


concealment against detection by means of electromagnetic 
radiations whose wavelengths lie either in the visible region 
of the spectrum or so closely adjacent thereto that the detec¬ 
tor is not radically altered by the extension of the wave¬ 
length range. 

12 RECOMMENDATIONS OF THE 
AD HOC COMMITTEE 

The ad hoc committee made the following report a 
and recommendations: 

The Committee has failed to find any problem or group 
of problems whose solution appears to depend upon an 
extension of existing knowledge in the sciences with which 
camouflage is concerned. It is, therefore, unwilling to rec¬ 
ommend that all research and development activities be 
concentrated in a single research laboratory created espe¬ 
cially for the purpose. 

This Committee believes, furthermore, that the prose¬ 
cution of the war effort is likely to handicap the research 
and development programs now conducted under Army or 
Navy cognizance because of increasing demands on the 
personnel in connection with both operations and training. 
In the main, optics in its physical and physiological aspects, 
photography, and certain branches of chemistry are the 
fields of science most concerned in the development of new 
camouflage techniques. It is common knowledge that there 
are many university and industrial laboratories possessing 
both adequate facilities and competent personnel in these 
fields, and that these facilities are not at present utilized to 
full capacity in the war effort. Since progress in the improve¬ 
ment of camouflage techniques involves a study of a large 
gamut of individual problems which are technically dissim¬ 
ilar and have only their major purpose in common, it would 
seem that the needs of the armed services can best be met 
by an arrangement under which each problem or closely 
related group of problems can be referred for solution to 
the proper university or industrial laboratory. 

The NDRC is uniquely organized to coordinate activi¬ 
ties under the above recommendation. The Committee 
therefore recommends the establishment of an NDRC Sec¬ 
tion on Camouflage. In view of the fact that the NDRC 
has already organized a Section on Illumination (C-6) 
which is concerned with the nocturnal aspects of camouflage, 
it is suggested that the Section on Camouflage be estab- 


a The report of the ad hoc committee contains five 
appendices of special interest: History and Literature of 
Camouflage; Camouflage Developments Under Army Cog¬ 
nizance; Camouflage Developments Under Navy Cogni¬ 
zance; Camouflage Developments Under Civilian Cogni¬ 
zance; and Bibliography. See reference 1. 


CONFIDENTIAL 


3 



4 


INTRODUCTION AND SUMMARY 


lished under Division C. To facilitate the desirable liaison 
between the two sections, the Chairman of each should 
preferably be made a member of the other. 

13 THE NDRC CAMOUFLAGE SECTION 

Upon the recommendation of the ad hoc commit¬ 
tee, the NDRC established a Camouflage Section. 
The new Section (C-8) was organized and, at its 
first meeting, agreed that as a matter of funda¬ 
mental policy its primary concern should be the 
camouflage of offensive weapons (ships, planes, 
tanks, etc.) rather than defensive camouflage against 
aerial bombardment, upon which most previous re¬ 
search effort had centered. Although this policy 
ultimately dominated the activities of the Camou¬ 
flage Section, its first efforts were directed toward 
completing certain researches in defensive camou¬ 
flage that had received the attention of some of its 
personnel before the Japanese attack on Pearl Har¬ 
bor. 

131 Camouflage Research Before 

Pearl Harbor 

Before the attack on Pearl Harbor, no research in 
camouflage was conducted under NDRC auspices. 
This was not the result of an oversight but a delib¬ 
erate policy which stemmed from the close connec¬ 
tion of certain NDRC personnel with an Army- 
sponsored civilian camouflage research organization 
known as the Passive Defense Project [PDPj. Oper¬ 
ated by funds from the Work Projects Administra¬ 
tion, the PDP conducted an extensive program of 
research in defensive camouflage. The researches 
described in Sections 1.3.2 and 1.3.3 were initiated 
by the PDP and were continued by the NDRC 
Camouflage Section. 

132 Chlorophyll Paint 

Research on a camouflage paint made from chlo¬ 
rophyll-bearing plant material had been conducted 
at Harvard University for PDP by Kenneth V. 
Thimann. Promising progress had been made on 
this project, and it appeared that a relatively small 
amount of additional work might produce a paint 
having the exact spectral characteristics of vegeta¬ 
tion. Recognizing that such a paint would make pos¬ 
sible the construction of detection-proof camouflage 
in vegetated areas, the Section placed a short-term 


contract (OEMsr-551) with Harvard University to 
enable Thimann to complete his research. The final 
results are embodied in Report on The Preparation 
and Properties of Chlorophyll Paints. 2 

No military application of chlorophyll paint is 
known to have been made. This was due partly to 
the fact that the procurement of camouflage paints 
by the Army was well under way, and partly to ex¬ 
perience in Europe and Britain which seemed to 
indicate that infrared-bright green paints made of 
chromium oxide and kindred materials afforded sat¬ 
isfactory concealment against the detection means 
employed by the Germans. 

1,3,3 Camouflage Design by Engineering 
Methods 

The course of World War II in Europe during 
1940 and 1941 caused the Army to begin laying plans 
for extensive camouflage installations designed to 
protect key factories and airfields from bombing at¬ 
tacks. PDP was charged with the creation of such 
designs. It became apparent that huge sums of 
money and large amounts of labor would be ex¬ 
pended on camouflage construction designed without 
knowledge of the optical requirements that must be 
met in order to achieve successful concealment. 
British experience had shown that trial-and-error 
methods often lead either to costly failures or to 
needlessly expensive successes. For this reason, it 
became the primary objective of the Physics Depart¬ 
ment of PDP to produce an engineering basis for the 
selection of camouflage materials. This required (1) 
laboratory instrumentation, (2) instruments for field 
use, and (3) data on the ability of atmospheric 
haze to obscure distant objects. 

Laboratory Instrumentation 

Infrared Spectrophotometer. An automatic re¬ 
cording photoelectric spectrophotometer 3 > 4 ’ 5 manu¬ 
factured by the General Electric Company was 
modified by PDP to extend its wavelength range to 
cover the near-infrared spectral region. Subsequently 
the Interchemical Corporation, a contractor of the 
NDRC Camouflage Section, made a like modifica¬ 
tion on its General Electric spectrophotometer. Data 
secured with the latter instrument played a promi¬ 
nent part in the researches supervised by the Cam¬ 
ouflage Section. 20 

Infrared Reflectometer. The use in aerial cameras 


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THE NDRC CAMOUFLAGE SECTION 


5 


of photographic film sensitive to infrared 6 > 7 made it 
necessary to evaluate the reflectance of camouflage 
materials in this spectral region. An instrument for 
this purpose was designed and nearly completed by 
PDP. Later, at the request of the Army, the instru¬ 
ment was completed by the electronics staff of the 
Research Laboratories of the Interchemical Corpo¬ 
ration under Contract OEMsr-697, supervised by 
the Section. The completed reflectometer was set up 
in the laboratory of the Materials Branch of the 
Engineer Board at Fort Belvoir, Virginia. Reports 
from that laboratory indicate that the instrument 
was in constant use throughout the war Figures 1 
and 2 are photographs of the reflectometer. It has 
been described in detail in the contractor’s report. 83-6 



Figure 1 . Infrared reflectometer. 


The sample to be measured is presented at the window of the 
integrating sphere in the upper center of the panel. The reflectance 
is indicated by an illuminated dial located at the left of the 
window. The instrument is self-balancing by means of the motor. 

The tube above the instrument permits adaptation for measuring 
transmissions as shown in Figure 2. 

Recording Goniophotometer. As viewed from the 
air. vegetated areas have goniophotometric proper¬ 
ties 9 > 10 which differ widely from those of ordinary 
flat surfaces. To facilitate an investigation of the 
corresponding properties of camouflage material, an 


automatic, recording, photoelectric goniophotometer 
was designed and partially completed by PDP. 
Later, at the request of the Army, the instrument 
was completed by the electronics staff of the Re- 



Figure 2. Infrared reflectometer arranged for trans¬ 
mittance measurements. 

A neon-crater lamp in the exterior tube is imaged by means 
of microscope objective to form a spot approximately 3/100 inch 
in diameter on the sample. The light transmitted by the sample 
is collected by the integrating sphere within the instrument. 
When used in this way, the reflectometer serves as a micro¬ 
densitometer. 

search Laboratories of the Interchemical Corpora¬ 
tion under Contract OEMsr-697, supervised by the 
Camouflage Section. 83-6 The completed instrument 
was delivered to the Materials Laboratory of the En¬ 
gineer Board, Fort Belvoir, Virginia. The recording 
goniophotometer, shown in Figure 3, can trace in a 
few minutes a complete curve showing reflectance 
as a function of angle, thus yielding data that for¬ 
merly required hours to obtain. Typical curves are 
shown in Figures 65 and 66 of Chapter 5. The instru¬ 
ment is described in OSRD Report No. 6556. 83 

Instruments for Field Use 

The Spectrogeograph. The plans of PDP included 
a number of instruments for field measurements, the 
most important of which was a spectrophotometer 
for aerial use. Although PDP had been unable to 
secure funds and materials to construct such an in¬ 
strument, the NDRC Camouflage Section placed a 
contract (OEMsr-717) with the Eastman Kodak 
Company, under which a special spectrograph 
adapted for aerial use was constructed. This instru¬ 
ment, called the spectrogeograph (see Figure 4), is 
described in Chapter 5 of this volume and in Report 


CONFIDENTIAL 











6 


INTRODUCTION AND SUMMARY 


No. 5444. 11 After the war had ended, the spectro- 
geograph was given by OSRD to the Naval Research 
Laboratory. 

Atmospheric Scattering of Light 

Since all camouflage measures are viewed through 
an intervening layer of atmosphere, the effects due 
to atmospheric scattering of light govern, in large 


senting the effect of the atmosphere along a vertical 
light path, and the other the effect along a horizon¬ 
tal light path. The first of these cases is of impor¬ 
tance in connection with the visibility from aircraft, 
and the second is typical of the case of a ship at 
sea viewed against the horizon. (See Chapters 4 and 
5 of this volume.) 

Both the Army (Project CE-24) and the Navy 



Figure 3. Recording goniophotometer. 

The sample to be measured (white rectangle, upper center) is illuminated at any selected angle by a collimated beam of modulated 
light from a neon-crater lamp in the tube supported by the fixed arm (left). The source spread is V 2 degree in order to simulate the 
geometry of sunlight. The sample is photometered by a type PJ14b photoelectric cell on a movable arm (right) which also carries a 
comparison lamp modulated with a phase opposite to that of the lamp in the fixed arm. A recording pen, attached to the same arm, 
automatically produces a polar plot of gonioreflectance on the graph paper carried by a platen (center) which is fixed with respect to the 
sample. The movable arm also carries a balance motor, Variac, and pen-positioning mechanism. 


measure, the color tolerance of camouflage require¬ 
ments. In order to estimate the magnitude of these 
tolerances, PDP devised and built a number of haze 
boxes similar to the one shown in Figure 5. These 
boxes were used for viewing and photographing ac¬ 
curately colored scale models. 

Two important phases of the study of atmospheric 
scattering may be separately identified: one repre- 


(Project NS-147) requested the Camouflage Section 
to make a quantitative study of the reduction of 
contrast due to atmospheric scattering. A prelimi¬ 
nary survey of existing experimental data and theo¬ 
retical treatments 12 of meteorological visibility dis¬ 
closed both a lack of data and the need for further 
development of the theory. 

A theoretical analysis of the effects along a hori- 


CONFIDENTIAL 

























THE NDRC CAMOUFLAGE SECTION 


7 



zontal path within the atmosphere was made 13 
(Section 2.2.1), and the results were expressed in a 
convenient form for use. For an experimental verifi¬ 
cation, a series of black and white targets was 
erected on the shores of Cold Spring Harbor, Long 
Island, New York, at ranges up to 6,000 yards. In 
order to determine the apparent contrast of these 





Figure 5. Schematic plan of the PDP haze box. 

The observer’s field of view is flooded with light from an 
illuminated blue-white screen by reflection from a glass plate. 
The amount of the simulated atmospheric haze can be adjusted 
by means of the rheostat. Lower sketch: Haze box in use by 
observer of model airfield. 


targets, a high-precision a-c photoelectric telepho¬ 
tometer was built (Section 2.2.3). Data secured with 
this instrument serve to support the theoretical 
analysis. Theoretical and experimental conclusions 
reached during this study were combined with data 
on the perceptual capacity of the human observer 
(Chapter 3) in a series of nomographic charts 
(Chapter 4) especially suited for use by ships at 
sea in predicting the limit of visibility of naval 
targets under the full range of outdoor lighting 
Figure 4. The spectrogeograph. conditions. 

An expedition was sent to Orlando, Florida, to 


CONFIDENTIAL 


























8 


INTRODUCTION AND SUMMARY 


secure a variety of data with the spectrogeograph, 
including data on the vertical scattering of light. 
A B-17, based at Orlando, was assigned to work in 
collaboration with the members of the expedition, 
which was under the direct supervision of one of 
the Section’s technical aides. By invitation, a repre¬ 
sentative of the U. S. Weather Bureau accompanied 
the expedition. A laboratory for processing the film 
and reducing the data was set up in Orlando, and 
a series of flights over a large gray scale laid out on 
the Orlando Army Air Base was made up to alti¬ 
tudes of 16,000 feet. An analysis of the data ob¬ 
tained during these flights enabled the construction 
of nomographic charts for predicting the visibility 
of objects on the surface of the earth as seen from 
the air (Chapter 5). 

Reflectivity of Natural Terrains 
The spectral reflectivity of natural terrains was 
measured by means of the spectrogeograph (Chap¬ 
ter 5). These data serve two purposes: (1) they in¬ 
dicate immediately the proper reflectivity for paints 
or other camouflage materials designed to match 
any type of terrain; (2) when combined with data 
on atmospheric scattering, they enable acceptable 
tolerances in the color match to be prescribed. 
Camouflage treatments designed in accordance with 
the results obtained under this project should 
therefore evade detection by the use of color filters, 
either visually or photographically. 

Field Studies in Florida and California. In Flor¬ 
ida, data 14 were obtained of such typical terrains 
as fields, forests of coniferous and of deciduous 
trees, lakes, rivers, roads, airports, and the ocean. 
The spectrogeograph was then flown to California. 
This flight was for the purpose of securing data on 
kinds of terrains not available in the East, espe¬ 
cially desert areas of different types, such as shift¬ 
ing sands, lava beds, dry lakes, and brush-covered 
areas; and also mountains, including both forested 
and snow-clad peaks. 

The Color of Ocean Shoals 
Prior to the departure of the expedition to Flor¬ 
ida, the Section was requested by the Coordinator 
of Research and Development, U. S. Navy, to supply 
data on the spectral quality of light reflected by the 
ocean in the vicinity of shoals. It was hoped that 
such data might point the way to the development 
of new photographic materials capable of showing 


the presence of shoals better than the film usually 
used for aerial reconnaissance. 

The measurements were made over a series of 
shoals which fringe the coast of Dania, Florida. A 
line of buoys was anchored perpendicular to the 
shoreline, soundings were taken, and samples of the 
bottom were obtained. Data on the reflectivity of 
the sea along this course were obtained with the 
spectrogeograph, both from an airplane and from 
a glass-bottomed boat. 15 These data, similar in 
trend, have suggested several improvements in pho¬ 
tographic techniques both in black-and-white and 
in special color photography. The data and the sug¬ 
gestions of the Section for improved techniques were 
furnished to the Navy, and it is understood that the 
Navy asked the Eastman Kodak Company to co¬ 
operate in the development and testing of new types 
of sensitized products for use in surveying under¬ 
water terrains. 

13,4 Color Transients 

Representatives of the Corps of Engineers re¬ 
quested the assistance of the Camouflage Section 
in exploring certain mysterious effects which were 
being reported by camouflage artists returning from 
California, where preparations were in progress for 
the impending battles on the African desert. It was 
reported that standard camouflage materials often 
behaved in an unexpected manner when seen against 
desert landscape, the camouflage being considered 
quite inadequate at certain times of the day. Per¬ 
sonnel and equipment, successfully camouflaged for 
most conditions in the desert, were occasionally seen 
with vivid color contrast against a typical desert 
terrain. Although the contrasts were reported to 
appear at any time of day, they most commonly 
were seen when the sun w r as low, in the early morn¬ 
ing or the late afternoon. These color transients 
were usually of short duration, at least in their most 
vivid phases. 

Project CE-26 

The representatives of the Camouflage Section 
who conferred with the Army engineers concerning 
the color transients expressed the opinion that they 
are attributable to the normal color changes that 
occur as a consequence of gross variation in the 
quality of the illumination. In the hope that proce¬ 
dures might be devised for the selection of camou¬ 
flage materials that would exhibit a minimum of 


CONFIDENTIAL 



THE NDRC CAMOUFLAGE SECTION 


9 


the transient effect, the Corps of Engineers re¬ 
quested NDRC to send a field expedition to the 
desert to study the transient phenomena. This re¬ 
quest bore A/N Project Control Number CE-26. 

The conclusions reached after the field expedition 
had returned from the desert are described in a re¬ 
port entitled Transient Color Phenomena in a Des¬ 
ert. 16 The conclusions presented therein are in gen¬ 
eral agreement with the earlier a priori opinions. 
The report also includes recommendations with re¬ 
spect to procedures in the selection of camouflage 
materials that should result in reducing the tran¬ 
sient effects to a minimum. 

1,3,5 Camouflage of Water Surfaces 

A method of camouflaging water surfaces (ponds, 
reservoirs, smooth rivers, etc.) by the use of thin, self¬ 
spreading and self-healing, pigmented films was sug¬ 
gested to NDRC by the American Cyanamid Com¬ 
pany. Since it was well known that bodies of water 
are among the principal visual aids to bombers in 
locating their targets, the Section explored the in¬ 
terest of the Army in camouflage measures for 
water surfaces. After being informally advised by 
an official representative of the Camouflage Section 
of the Engineer Board, Fort Belvoir, that the sub¬ 
ject of water camouflage was of definite interest to 
the Army, and after a survey of potential contrac¬ 
tors had disclosed that the American Cyanamid 
Company, Stamford, Connecticut, was best suited 
to conduct the needed research, an OSRD contract 
(OEMsr-726) was placed with this organization. 
Self-spreading, self-healing films composed of 
treated woodchips were produced and tested on 
ponds (see Figure 6). Later, a powder of sulfur and 
polystyrene was developed which remains on the 
surface indefinitely and forms a self-spreading film 
which may be of any color. A pound of this mate¬ 
rial completely obscures 300 sq ft of water surface 
at an estimated cost of about 8 cents. These mate¬ 
rials were tested both on fresh and salt water. The 
results of the experiments are described in Report 
on Water Camouflage . 17 

1 ‘ 3 ' 6 Camouflage Paints 

The activities of PDP thoroughly acquainted the 
Army and most manufacturers of camouflage paint 
with the problem of matching the spectral charac¬ 


teristics of vegetation, both in the visible region of 
the spectrum and in the near infrared. However, 
much valuable research on camouflage paints re- 



Figure 6. The nearby portion of this pond is covered 
by a self-spreading camouflage film. Note the specu¬ 
lar reflection from the untreated portion of the pond. 


mained to be done in 1942, when the NDRC Cam¬ 
ouflage Section was organized. The analysis of the 
situation by the new Section showed that: 

1. A laboratory should be found capable of mak¬ 
ing small quantities of special paints for field ex¬ 
periments by other contractors of the Section. 

2. Competent research chemists should be given 
an opportunity to produce improved camouflage 
materials. 

Subsequent discussions with the Corps of Engi¬ 
neers led to a request (Project CE-25), called 
“Camouflage Paints and Pigments,” in which the 
Army asked that these facilities be provided. 

Proposal to Simplify Palette 

The Section proposed that an attempt be made 
to reduce from nine to four the number of standard 
camouflage paints supplied to troops in the field. 
This suggestion was based on the belief that any 
color in the required gamut could be produced by 
mixtures of three colored paints, and a fourth paint 
could be used to secure any desired degree of infra¬ 
red reflectance. A Section member reported that re¬ 
search on a tricolor palette for artists had already 
been conducted by the Research Laboratories of 
the Interchemical Corporation in New York City. 
Since the research laboratory of this company was 


CONFIDENTIAL 





10 


INTRODUCTION AND SUMMARY 


one of the few research groups in the country pos¬ 
sessing spectrophotometric equipment, a contract 
(OEMsr-697) with the Interchemical Corporation 
was drawn up whereby this laboratory could serve 
both of the purposes mentioned earlier. 

Summary of Completed Paint Projects 

Throughout the activities of Section 16.3, bi¬ 
monthly summary reports were issued in accord¬ 
ance with NDRC policy. The last report has been 
used as a pattern for the following summary of the 
camouflage paint projects completed by Section 16.3 
of NDRC. In all cases, the projects are fully dis¬ 
cussed in the contractor’s report. 18 

Matte Surface Paints 

Light incident upon natural terrains, such as 
grasslands and woodlands, is mostly trapped by 
the “texture” of the surface. The small fraction of 
light reflected from such terrains has a goniophoto- 
metric distribution quite unlike that of painted sur¬ 
faces. Therefore, camouflage treatments often in¬ 
volve elaborate texturing procedures such as the 
erection of flat tops having a texture introduced by 
garnishing. At the instance of the Section, the Inter¬ 
chemical Corporation evolved a method of pig¬ 
mentation enabling a paint to be produced having 
colorimetric and goniophotometric properties ap¬ 
proaching those of natural terrains. Because of 
special interest expressed by the Camouflage Section 
of the Engineer Board, Fort Belvoir, Virginia, in a 
green paint having the appearance of a textured 
surface such as grass or moss, a paint of the new 
type was developed which possesses many of the 
desired optical properties (Section 5.4.3). 18a 

Emulsifiable Paints 

The desirability of reducing the shipping weight 
and bulk of general utility camouflage paints for 
field use has been recognized by the development of 
paints employing an emulsifiable, oleoresinous vehi¬ 
cle that can be thinned in the field, either with water 
or with gasoline. When the Camouflage Section of 
NDRC was organized, emulsifiable paints were only 
beginning to be used extensively, and many troubles 
were encountered. At the request of the Engineer 
Board at Fort Belvoir, Virginia, the Section had the 
Interchemical Corporation investigate the rheo¬ 
logical properties of pigmented emulsions. Satisfac¬ 
tory emulsifiable paints are now being supplied to 


the Armed Forces in conformity with present Army 
specifications. 1815 

Paint Concentrates 

The Interchemical Corporation formulated a 
paint that can be shipped in powder form and mixed 
either with water or with gasoline. If mixed with 
gasoline, the paint is readily removable with gaso¬ 
line; and, consequently, can be used where tempo¬ 
rary, easily removable paints are desired. The Mate¬ 
rials Laboratory at Fort Belvoir, Virginia, has 
advised, however, that the advantages to be gained 
by a powder paint of this sort are not great enough 
to warrant their substitution for the emulsifiable 
paints now employed by the Services. 1815 

Foliage-Simulating Pigments 

Most common green pigments, which appear on 
visual examination to match the color of chloro¬ 
phyll, are open to the objection that camouflage 
using them is readily detected by infrared photog¬ 
raphy. This difficulty can be met by using paint in 
which chlorophyll itself is used as a coloring mate¬ 
rial (Section 1.3.2). However, before recommending 
the large-scale development of such a paint, the 
Camouflage Section addressed letters to all the prin¬ 
cipal manufacturers of colored pigments and secured 
approximately one hundred samples for spectropho¬ 
tometric analysis. These samples were coded and 
tested under standard procedures designed to eval¬ 
uate the relevant optical properties and the perma¬ 
nency of the pigments. The results of these tests are 
presented in the final report of the contractor. 180 

Temperature-Sensitive Pigments 

Materials exhibiting reversible color changes in 
response to changes in temperature might have uses 
in camouflage paints. For instance, a paint which 
changes from dark green to white within the range 
of temperature between summer and winter might 
find applications in certain latitudes. A survey was 
made of temperature-sensitive materials, the mech¬ 
anism of color change was studied in specific cases, 
and new materials were synthesized which exhibit 
temperature-sensitive properties. Efforts to decrease 
the temperature range in which the color changes 
take place were unsuccessful. 1811 

Camouflage Adhesive 

Natural materials available in the field are often 
useful for camouflage purposes. To cover smooth 


CONFIDENTIAL 




THE NDRC CAMOUFLAGE SECTION 


11 


surfaces with materials such as dirt, leaves, grasses, 
etc., an adhesive is required that is water resistant 
yet readily removable, capable of being modified to 
suit the field conditions, and readily available. At 
the instance of this Section, a material was formu¬ 
lated that is tacky, water resistant, and inexpensive. 
This material can be thinned with gasoline to the 
appropriate consistency before application, and its 
tackiness can be controlled by the addition of cyl¬ 
inder oil or crankcase oil. The adhesive can be re¬ 
moved with gasoline. 186 

Claray Project 

Scales and reticles of optical instruments for fire 
control and other purposes are often illuminated by 
an integrating cavity containing a small lamp. In 
such a cavity, a white paint reflecting 98 per cent 
is twice as efficient as one reflecting 96 per cent, al¬ 
though this difference in reflection factor is barely 
discernible when the two paints are compared vis¬ 
ually. The Navy Bureau of Ordnance sought advice 
from this Section concerning the possibility of se¬ 
curing white paints of higher reflectivity than are 
commercially available. The Interchemical Corpo¬ 
ration was known to have experimented with a 
material called Claray, which has a reflectance rela¬ 
tive to magnesium oxide of 95 per cent. Subsequent 
work produced a product having a reflectance of 
98 per cent; and samples were furnished to the 
Bureau of Ordnance and elsewhere. 18f 

Coffin Paint 

A new type of black pigment that requires no 
flatting agent was developed and incorporated in a 
paint which is extremely matte and which reflects 
only 2.2 per cent of the incident light instead of the 
4 or 5 per cent characteristic of standard-type matte 
black paint. This paint was tested as an antisearch¬ 
light camouflage measure for aircraft, but was found 
to be less effective than the antisearchlight camou¬ 
flage described in the next paragraph. However, it 
appears that this “coffin paint” may be useful in the 
simulation of shadows on the ground; and at the 
request of the Materials Branch of the Engineer 
Board at Fort Belvoir, Virginia, information was 
furnished that would enable an Army procurement 
specification to be written. 186 

Antisearchlight Camouflage 

It is a fact of common experience that even black- 
painted aircraft look white when caught in search¬ 


light beams. The inference to be drawn from such a 
statement is that, if the visibility of aircraft is to 
be reduced, there must be a reduction of at least an 
order of magnitude in the diffuse reflectivity of 
conventional matte black finishes. Such a reduction 
has riot been found possible with a matte surface, 
but it has been accomplished with a glossy surface. 
A glossy black enamel was developed, whose dif¬ 
fuse reflectance is less than 0.1 per cent. Model 
trials at the Tiffany Foundation and Service tests at 
Eglin Field, Florida [see AAF Proving Ground 
Command Report Serial No. 3-43-111, AAF Board 
Project No. (M-l) 17] indicated a high degree of 
success in rendering the camouflaged plane invisible 
in searchlight beams and in making it extremely dif¬ 
ficult for searchlight operators to fix and hold on 
the plane. Orders issued from the commanding 
general, AAF, required all U. S. night fighters to 
use this camouflage, and procurement specifications 
were subsequently issued by the AAF Materiel 
Command at Wright Field. At the request of the 
Army Air Forces, specialists were sent by the OSRD 
Office of Field Service to the Fifth, Eleventh, Thir¬ 
teenth, and Fourteenth Air Forces to supervise the 
application of this finish. The Assistant Chief, Mis¬ 
cellaneous Section, Proving Ground Command, su¬ 
pervised the application of the antisearchlight cam¬ 
ouflage to aircraft of the Eighth Air Force in Eng¬ 
land and demonstrated it to the RAF. He also vis¬ 
ited all U. S. Army Air Forces engaged in the war 
with Japan, where he introduced antisearchlight 
camouflage and supervised its initial application. 
Tests of this camouflage measure were conducted by 
the Navy Department, Bureau of Aeronautics, at 
the Naval Air Station at Patuxent River, Mary¬ 
land. A final report, Tests of Jet Paint Night Cam¬ 
ouflage, concurs with the favorable findings de¬ 
scribed in the Eglin Field report, and adds that the 
antisearchlight camouflage is never more visible 
than is matte black under moonlight or starlight. 

The Black Widow finish, as it came to be called, 
was widely used both in the European theater of 
operations and in the Pacific war. It is understood 
that this camouflage was adopted in production by 
the RAF during the closing months of the war and 
that during the same period B-29’s were being pro¬ 
duced with Black Widow finish. It is further under¬ 
stood that the Army Air Forces have issued stand¬ 
ing orders that all military combat aircraft, includ¬ 
ing both fighters and bombers, intended for night- 


CONFIDENTIAL 




12 


INTRODUCTION AND SUMMARY 


time operation shall be equipped with Black Widow 
finish. Chapter 7 of this volume is devoted to the 
Black Widow project. 1811 

Self-Luminous Night Camouflage 

At the time of the reorganization of NDRC, an¬ 
other section of NDRC had proposed the use of self- 
luminous paint as a means of enabling aircraft to 
match the brightness of the night sky. Exploratory 
conversations with Army and Navy personnel 
evoked only slight interest in this project, presum¬ 
ably because it did not appear feasible to combine 
the characteristics of a self-luminous paint with 
those of the antisearchlight paint. 

1,3/7 Camouflage Detection 

Bifocal Goggles 

Dichroic filters were developed during World 
War I as a means for differentiating actual foliage 
from camouflage materials of the same color. Such 
filters can cause foliage to appear red, whereas or¬ 
dinary green paints remain green. Fliers ordinarily 
dislike wearing dichroic goggles because so much of 
the land surface is covered by vegetation that the 
appearance of the earth from an airplane is un¬ 
natural. On the other hand, unless dichroic goggles 
are worn continually, a period of time is required 
for the eyes to adapt themselves after putting on 
the goggles, due to their low transmittance. Bifocal 
goggles with a dichroic filter in the lower half of 
the lens and a neutral filter having the same trans¬ 
mittance. in. the upper half were proposed by the 
Section in order that these may be worn contin¬ 
ually and the dichroic feature may be instantly 
brought into use at any time. A number of such bi¬ 
focal dichroic goggles were made available to 
interested Army and Navy personnel. 

Optical Aids for the Detection of Submerged 
Submarines 

At the request of the Navy, the Section has re¬ 
viewed all available optical aids for the detection 
of submerged submarines. Two series of special 
filters based on the known optical properties of sea 
water have been obtained. One series is intended to 
increase the contrast of a submerged object that is 
darker than the sea beneath it, while the second 
series of filters is intended to increase the apparent 
contrast when the submerged object is lighter than 


the sea beneath. These filters have been mounted in 
special goggles containing rotatable polaroid screens 
and provided with means for limiting the field of 
view in order to reduce the glare from the surround¬ 
ing field. These goggles were tested by the Navy, 
and it was concluded that they did not produce a 
sufficient improvement in the visibility of sub¬ 
merged craft to warrant their adoption, in view of 
other methods of submarine search now available. 

1,3,8 Laboratory for Camouflage 
Field Studies 

In the belief that facilities would be required for 
making camouflage studies in the field, the Camou¬ 
flage Section placed a contract (OEMsr-597) with 
the Louis Comfort Tiffany Foundation, Oyster Bay, 
New York. Actually, this contract served only to 
defray the special operating expenses of such field 
experiments, for the Tiffany Foundation generously 
gave NDRC the use of its 80-acre estate on Cold 
Spring Harbor without cost to the Government. An 
art school during the years of peace, the Tiffany 
Foundation faced the prospect of having its facili¬ 
ties lie unused during the war. It, therefore, offered 
them first to the Navy and later to NDRC. After an 
inspection of the estate had indicated that the 
grounds at Oyster Bay offered an admirable location 
for camouflage field studies, the above-mentioned 
contract was negotiated. 

Facilities of the Tiffany Foundation 

The 80-acre estate bordering on the shore of Cold 
Spring Harbor contains a wide variety of terrains, 
including ponds, lawns, and wooded hills. Adjacent 
to the property of the Foundation lay the 300-acre 
estate of Charles Tiffany, who generously permitted 
certain tests to be performed on his land when it 
proved to be more suitable than any owned by the 
Foundation. The studio buildings, which had been 
occupied by the art school, provided ample living 
and working facilities for the experiments required 
by the Section. The major portion of the researches 
described in this volume was performed there. 

0 

Use by the Armed Forces 

On several occasions the facilities of the Tiffany 
Foundation were made available to the Armed 
Forces for special tests. The following case is an 
example. 


CONFIDENTIAL 




THE NDRC CAMOUFLAGE SECTION 


13 


Recognition Threshold of Colored Lights. The 
Navy Bureau of Ships requested this Section to col¬ 
laborate in a study of the distance at which the color 
of colored lights can be recognized at night under 
conditions of poor visibility. Colored lamps pro¬ 
vided by the Navy were set up on the shores of Cold 
Spring Harbor. The lights were viewed from 3,660 
yards by a group of Tiffany observers on dark 
nights under weather conditions such that the range 
of visibility was considerably restricted. The inten¬ 
sity of the lights was varied in accordance with in¬ 
structions communicated with the aid of police radio 
cars. A report incorporating the results obtained 
during these experiments has been issued by the 
Bureau of Ships. 

1,3 9 Ship Camouflage 

Wake Camouflage 

Even at low speed, a motor torpedo boat is con¬ 
spicuous because of its bow wave and wake. Experi¬ 
ments conducted informally by the American Cy- 
anamid Company have indicated the feasibility of 
spraying suspensions of carbon black from bow and 
stern in such a manner as to conceal the white 


water. The Navy Department purchased the equip¬ 
ment for further tests, and asked this Section to 
consider the possibility of using the Tiffany Foun¬ 
dation at Oyster Bay as a base. Investigations dis¬ 
closed a nearby boat yard where PT boats fre¬ 
quently are repaired. The Section reported to the 
Coordinator’s Office that it had suitable facilities at 
its disposal, and indicated its willingness to under¬ 
take the tests. However, a request for this work was 
not made by the Navy Department, and work on 
this project was not begun. 

Model Trials 

The Bureau of Ships requested that observations 
of ship camouflage be made under natural outdoor 
conditions on ship models in order to test the rela¬ 
tive merits of camouflage designs. Two identical 
20-foot cruiser models (Figure 7) were delivered by 
the Navy to the Tiffany Foundation at Oyster Bay 
where a boathouse and a marine railway were con¬ 
structed. At a short distance from shore, a platform 
was erected from which to observe the models 
through an inverted periscope in order to simulate 
observations from a submarine. The periscope and 
a range finder were‘supplied by the Navy, and in- 



Figure 7. A 20-foot cruiser model used in Cold Spring Harbor. 


CONFIDENTIAL 






14 


INTRODUCTION AND SUMMARY 


struments were procured for determining the me¬ 
teorological conditions, including the visibility, at 
the time observations were made. A series of ob¬ 
servations resulted in the conclusion that the counter 
shading of contained shadows is ineffectual in the 
case of a medium-gray ship observed down-sun in 
clear weather. Following the destruction of the ob¬ 
serving platform, the boathouse, and one of the ship 
models by a tropical hurricane, the Bureau of Ships 
recommended the termination of this project. 

1310 The Yehudi Project 

When the menace of German submarines to allied 
Atlantic shipping constituted one of the major prob¬ 
lems of World War II, the Camouflage Section was 
requested by the Director of Technical Services of 
the Army Air Forces to devise a method of camou¬ 
flage which would enable a sea-search aircraft to 
approach within 30 seconds’ flying time of a sur¬ 
faced submarine before the aircraft became visible 
to members of the U-boat crew. Such an aircraft 
when flying at low elevations over water appears 
darker than the sky background, even when painted 
white. Calculations showed that the power required 
to eliminate the contrast by floodlighting the plane 
is prohibitive. 

The solution to the problem proposed by the 
Camouflage Section involves the installation of au¬ 
tomobile headlights, or the equivalent, in or near 


the leading edge of the wings and in the fuselage. 
It was pointed out that a minimum amount of power 
would be required when the headlights are designed 
to include the smallest angle consistent with the 
pitching and yawing of the plane. 

Project AC-45 

In response to a formal request from the Army 
Air Forces (A/N Project Control No. AC-45), the 
Section instructed the laboratory group at Tiffany 
to explore the suggested camouflage measure. The 
first experiments were at model scale, but later the 
principle was tested on a full-scale model of a B-24 
airplane. The Yehudi project, as it came to be 
called, is described in detail in Chapter 6 and in 
three contractors’ reports which appear in the mi¬ 
crofilm supplement. 19 ’ 20 * 21 The information ob¬ 
tained by the contractors enabled the Army Air 
Forces to design and install Yehudi camouflage on 
a B-24 bomber. Figures 8, 9, and 10 are Army Air 
Forces photographs of this installation. 

Project NA-188 

Camouflage of a Torpedo Bomber. Section 16.3 
was requested by the Navy to assist in applying the 
Yehudi camouflage principle to TBF torpedo bomb¬ 
ers. The first flight test of a TBF so equipped occa¬ 
sioned favorable reaction. Further flight tests re¬ 
sulted in improvements in the adjustment of the 
equipment and in the techniques for its use. It is 



rr 


Figure 8. B-24 bomber equipped with Yehudi camouflage. (Army Air Forces photograph.) 

CONFIDENTIAL 

















THE NDRC CAMOUFLAGE SECTION 


15 



understood that under conditions such that an un¬ 
camouflaged plane was visible at about 12 miles, 
the plane equipped with this camouflage could ap¬ 
proach to within 3,000 yards without detection, 
even when its approximate location had been indi¬ 
cated by an accompanying “control” plane. 

Camouflage of a Glomb. The Navy later re¬ 
quested Section 16.3 to design and install Yehudi 
camouflage on the LBE Glomb produced by the 
Gould Aeronautical Division of Pratt, Read & Com¬ 
pany, Incorporated, Deep River, Connecticut. An 
OSRD contract was placed with this company in 
order that one of the experimental LBE Glombs 
being built under a Navy contract might be factory- 
equipped with Yehudi camouflage. The end of hos¬ 
tilities with Japan caused the Navy to terminate 
the contract under which the Glombs were being 
built, thus making it impossible for Pratt, Read & 
Company to complete this project. (See Chapter 6.) 

1,311 Visibility of Targets 

On December 30, 1942, the Section met to discuss 
camouflage problems of interest to the Navy. This 


meeting was attended by Navy officers from the 
Bureau of Ships and the Bureau of Aeronautics 
who were interested in the visibility of ships and 
of aircraft. It was the consensus of the Section that 
it would be possible to eombine existing information 
on the perceptual capacity of the human observer 
with the known laws of atmospheric optics in such 
a manner that charts and tables for indicating the 
visibility of naval targets could be prepared. At a 
conference called by the Navy Department in 
Washington, the Section Chief was asked to under¬ 
take the preparation of such a set of charts and 
tables. The Navy formalized its action by request¬ 
ing NDRC, under A/N Project Control No. NS-147 
(Ship Camouflage), to undertake the study of the 
visibility of ships from other ships, ships from 
planes, planes from other planes, and planes from 
ships. The request was endorsed by the Bureau of 
Ships, Bureau of Aeronautics, and Bureau of Ord¬ 
nance, the last requesting that consideration be 
given to the effect of the use of binoculars. 

After a search of the literature had disclosed that 
usable' data on the perceptual capacity of the human 
observer were not available, a large-scale program 


Figure 9. Yehudi lamps mounted in the leading edge of the wing between the motor nacelles. (Army Air Forces 
photograph.) 


CONFIDENTIAL 







INTRODUCTION AND SUMMARY 


16 



Figure 10. Yehudi lamps in streamline housings suspended below the wing. This method of mounting was found 
to be permissible on the outer portion of the wing. (Army Air Forces photograph.) 


of visibility research was initiated by the Sec¬ 
tion. 22 ’ 23 ’ 24 Chapters 2, 3, 4, and 5 of this volume 
present a collation of the results of those researches 
by the Section and its contractors which now enable 
the visibility of targets to be predicted. 


14 ORGANIZATION OF THE SUMMARY 
TECHNICAL REPORT 

As explained in the preface of this volume, most 
of the projects which have been described in the 
foregoing summary have been completely discussed 
in the reports of contractors. However, the research 


on the visibility of targets was a Section program, 
the contractors being requested to obtain certain 
specific data. Responsibility for the direction of the 
research was assumed by the Section, and Section 
personnel collated the results after the contractors 
had finished their work. Since this synthesis does 
not appear elsewhere, it is presented in full in the 
following four chapters. 

Two aircraft camouflage measures, the Yehudi 
project and the Black Widow project, are believed 
to be the outstanding camouflage contributions of 
the Section. Chapters 6 and 7 are devoted to more 
detailed accounts of these measures than appear 
in the foregoing summary. 


CONFIDENTIAL 






PART II 


VISIBILITY OF TARGETS 















Chapter 2 

THE SCREENING OF TARGETS BY THE ATMOSPHERE 


INTRODUCTION 

A ll camouflage measures are viewed through a 
. veil of atmospheric haze which, by reducing 
the apparent contrast of distant objects, aids the 
camoufleur in making even large targets invisible. 
This subject falls naturally into two categories 
which will henceforth be referred to as visibility 
along a horizontal path and visibility along a slant 
path, respectively. In the former category lie most 
problems of ship camouflage, including the impor¬ 
tant case of visibility through a periscope. In the 
second category are the many problems of camou¬ 
flage against aerial observation and photographic 
reconnaissance. The basic principles are the same 
in both cases; but, because of the stratification of 
the atmosphere, the resulting laws are somewhat 
different. Visibility along a horizontal path can be 
regarded as a limiting case of visibility along a 
slant path; but the horizontal case will be treated 
independently because it affords a simple illustra¬ 
tion of the principles. 

22 THE VISIBILITY OF OBJECTS 
VIEWED ALONG A HORIZONTAL PATH 

If a large, nearby, white object illuminated by 
full sunlight is viewed against the horizon sky, it 
usually appears bright in comparison with its sky 
background. If the distance between the object and 
the observer is increased, the contrast between the 
white object and the horizon sky decreases. Indeed, 
at some range, dependent upon the state of the at¬ 
mosphere, this contrast may fall so low that the 
object is lost from view, even though it still sub¬ 
tends a large angle at the eye of the observer. The 
object may be said to be obscured by haze. 

For example, imagine sister ships, both painted 
white, viewed from such a vantage point that one 
is seen nearby and the other at a considerable dis¬ 
tance. If there is a slight haze, the distant vessel 
may appear but slightly brighter than the sky. 
Since the apparent sky background is identical for 
the two vessels, it is evident that some of the light 
reflected by the more distant ship has been atten¬ 
uated in passing from the vessel to the observer. 


In the case of black vessels, an opposite type of 
change takes place. The nearer ship appears very 
dark against the sky background, whereas the ship 
seen at a distance is but slightly darker than the 
sky. Since the sky background is the same for both, 
it is evident that the distant ship appears brighter 
because of light scattered toward the observer by 
the intervening air. It may be inferred from these 
two limiting cases that two processes are taking 
place simultaneously within the atmosphere: (1) 
light reflected by the target is gradually attenuated 
by scattering and absorption, and (2) daylight is 
scattered toward the observer all along the line of 
sight. 

2,2,1 Quantitative Relations 

The appearance of a distant object is governed 
by the balance between the transmitted fraction of 
the light originally reflected from the object and the 
space light contributed by the intervening air. An 
insight into the relation between these components 
can be gained in the following manner: Assume 
that the object in Figure 1 has a brightness B 0 in 



Figure 1 . See text for explanation. 


the direction of the observer. At some distance x 
imagine a parallel-sided flat lamina of atmosphere 
having a thickness dx to be located perpendicularly 
to the line of sight. Denote the amount of light (in¬ 
cluding both the transmitted component and the 
space light) incident from the left on each unit 
area of the lamina by t. In passing through dx, the 
attenuation is proportional to the amount of light 
present, the constant of proportionality /? being 


CONFIDENTIAL 


19 






20 


THE SCREENING OF TARGETS BY THE ATMOSPHERE 


called the attenuation coefficient. This coefficient ac¬ 
counts for diminution by absorption as well as for 
diminution by scattering. 

In passing through dx, the light t is also aug¬ 
mented by the space light contributed by the air 
within the lamina, as shown by equation (1): 


dt 

dx 


—fit + <*q, 


( 1 ) 


where q represents the luminous density at the lam¬ 
ina, and o is the fraction thereof that is scattered 
each second through the right-hand boundary 
(Figure l). a 

Equation (1) can be integrated along the line of 
sight after it has been rewritten in the form of equa¬ 
tion (2). 



( 2 ) 


In equation (2), B x is the apparent brightness of the 
object when viewed from a distance X. The result 
of the integration is shown in equation (3). 


, — fiB x 

11 oq - fiB 0 


fiX. 


Equation (3) may be rewritten as follows: 


(3) 


Bx = (^) ( 1 - «-») + B„e-ex. (4) 

The apparent brightness of an object at range X 
is shown by equation (4) to be the sum of two 
terms; the first represents the space light contrib¬ 
uted by the air between the target and the observer, 
and the second represents that fraction of the light 
originally leaving the target which is transmitted 
by the atmosphere. 


Optical Equilibrium 

It will be noted from equation (1) that whenever 
fit = oq, dt/dx — 0. Hence, under these circum¬ 
stances t has a constant value that does not depend 
upon X. In other words, when the light incident 
from the left on the lamina in Figure 1 has the value 


the attenuation by the lamina is equaled by the 
added space light, so that the amount of light emerg¬ 
ing through the right boundary of the lamina (Fig¬ 
ure 1) is also t'. This condition has been called op¬ 
tical equilibrium. 

The Brightness of the Horizon. In the special 
case of an optically homogeneous atmosphere, by 
which is meant an atmosphere wherein fi, a, and q 
have the same values at all points along the line of 
sight, an object having an inherent brightness 
B 0 = t' will appear to have the same brightness 
when viewed from any distance. 

Under optically homogeneous atmospheric condi¬ 
tions, the apparent brightness of the sky at the 
horizon in any given direction is not changed by 
moving toward the horizon or away from it. This 
observation implies that the brightness of the hori¬ 
zon ( B h ) is the equilibrium value t'\ that is, 

B„ = (6) 

Thus, under optically homogeneous atmospheric 
conditions, the brightness of the horizon sky is de¬ 
termined by a, q, and fi in accordance with equation 
(6). A discussion of the brightness of the horizon 
sky under certain types of optically nonhomoge- 
neous atmospheric conditions appears in Section 
2 . 2 . 6 . 

Extent of Optical Equilibrium. An estimate of 
the length of the path of sight along which optical 
equilibrium may be assumed can be obtained from 
calculations based upon equation (4). The result 
of such a calculation, assuming a homogeneous 
standard atmosphere (Section 2.3.2), is shown in 
Figure 2. It will be noted that the region of optical 
equilibrium occupies a range of many miles, several 
times the meteorological range (Section 2.2.5). 

Apparent Brightness of Distant Objects 

Under optically homogeneous atmospheric condi¬ 
tions, the apparent brightness of a distant object is 
given by equation (7), which was obtained by sub¬ 
stituting equation (6) in equation (4). 


a Since |3 and a may be functions of wavelength, equation 
(1) is strictly true only if the light is monochromatic. The 
experimental results described in Section 2.2.3 and the results 
of other investigators seem to justify the use of equation 
(1) in discussing the screening of targets by the atmosphere. 


B x = B h ( 1 - e-fK) + B 0 e-PX. (7) 

The Transmittance of the Atmosphere. As has al¬ 
ready been stated, the term B 0 e'P x in equation (4) 
represents that portion of the light originally leav¬ 
ing the target which is transmitted by the atmos- 


CONFIDENTIAL 






THE VISIBILITY OF OBJECTS VIEWED ALONG A HORIZONTAL PATH 


21 



Distance along Horizontal Plane in Miles 

Figure 2. Apparent sky brightness along a path tangent to the surface of the earth on a very clear day when the 
meteorological range (Section 2.2.5) is 60,000' yards. 


phere. The transmittance T x of a path of length X 
is therefore 

T x = e-/«. (8) 

The attenuation coefficient /3 can be determined 
with the aid of equation (8) from measured values 
of T x . For this purpose equation (8) can be re¬ 
written 

* 2.303 1 1 

P — logio jr • (9) 

A photoelectric transmissometer for measuring T x 
has been developed by the National Bureau of 
Standards. 25 

The transmittance T of a unit distance (mile, 
yard, etc.) of atmosphere is shown by equation (8) 
to be 

T = e~P. (10) 

By substituting equation (10) in equation (7), the 
latter can be written 

B x = B h {1 — T x ) + B 0 T X . (11) 

The Attenuation of Contrast 

Within the region of optical equilibrium, the 
brightness of a distant object can be computed by 


means of equation (4), provided the brightness of 
the object at zero range and the atmospheric atten¬ 
uation coefficient are known. However, the effect of 
atmospheric scattering on the visibility of distant 
targets can be represented more simply by rewrit¬ 
ing equation (4) in terms of the contrast of the 
object against its sky background. Let the apparent 
contrast of a target seen at a distance X against a 
background of horizon sky be defined by the relation 

(12) 

Similarly, let the inherent contrast of the target (as 
seen nearby) be defined by the relation 

C„=^=^. (13) 

As a consequence of the foregoing definitions, the 
contrast of dark targets can never exceed minus 
one, while the contrast of bright targets is unlim¬ 
ited. As will be shown in Chapter 3, dark targets 
and bright targets of the same size and having nu¬ 
merically equal apparent contrasts are equally 
visible. 


CONFIDENTIAL 









22 


THE SCREENING OF TARGETS BY THE ATMOSPHERE 



Figure 3. Variation of apparent contrast with distance for targets seen against a background of horizon sky on a 
day when the meteorological range is 12,000 yards. 



Figure 4. Sketch of Cold Spring Harbor, Long Is¬ 
land, showing the location of targets. 


By substitution, equation (7) takes on the form 
Cjr = C 0 6-^. (14) 

Similarly, equation (11) becomes 

C x — C 0 T X . (15) 

Equation (14) states that the apparent contrast 
of any target, bright or dark, is exponentially at¬ 
tenuated with distance. This is illustrated in Figure 
3, which shows the contrast attenuation with dis¬ 
tance for both black and white targets on a day 
when the meteorological range (Section 2.2.5) is 
12,000 yards; a bright target, having inherent con¬ 
trast greater than unity, is visible at greater dis¬ 
tance than the darkest dark target of equal size. 

2 2 3 Experimental Verification of the 
Theory 

The Tiffany Foundation, under contract OEMsr- 
597, performed experiments designed to test the 
theory developed above. A series of black and white 
targets was set up on the shores of Cold Spring 
Harbor at the locations shown in Figure 4. These 
were billboard-type structures, carefully placed so 


CONFIDENTIAL 









THE VISIBILITY OF OBJECTS VIEWED ALONG A HORIZONTAL PATH 


23 


that the angle of incidence of the sunlight was the 
same on each and adjusted in size so that every tar¬ 
get subtended the same angle at the observing sta- 



Figure 5. Billboard-type targets used by the Tiffany 
Foundation. 


tion on the Tiffany beach. Photographs of these 
targets are shown in Figure 5. The apparent con¬ 
trast of the targets relative to the horizon sky was 


measured by means of a telephotometer along the 
sight paths indicated by broken lines in Figure 4. 
Meteorological data of the conventional type were 
taken at the time of each experiment; and, in addi¬ 
tion, the transmission of the atmosphere over a 
1,000-yard path across Cold Spring Harbor was 
determined by means of a Bureau of Standards 
transmissometer. 

The simplest type of telephotometer used to 
measure the apparent contrast of the targets was 
the long-focus camera shown in Figure 6. A lens 
having a focal length of 10 feet was used in order 
to obtain an image of the distant billboard suffi- 



Figure 6. Long-focus camera used as a telephotom¬ 
eter. 


ciently large to permit reliable density measure¬ 
ments to be made with the microdensitometer de¬ 
scribed in Section 5.3.1. A photographic gray scale, 
mounted a few feet in front of the camera, provided 
sensitometric calibration on each negative. 

Photographs of the three targets on a clear day 
are shown in Figure 7. The clearness of the atmos¬ 
phere on this occasion can be judged by the fact 
that hills in Connecticut, 50 miles distant, are 
clearly shown in the third picture. The negatives 
from which these pictures were made were meas¬ 
ured with the microdensitometer, and the contrast 
of the target relative to the sky above the Connecti¬ 
cut hills was determined by the usual methods of 
photographic photometry. 26 Figure 8 is a semiloga- 
rithmic plot of apparent target contrast as a func¬ 
tion of range. In making this plot, no distinction 
has been made between positive and negative values 
of contrast. It will be noted that the three points 
representing the black targets fall along a straight 


CONFIDENTIAL 










24 


THE SCREENING OF TARGETS BY THE ATMOSPHERE 




Figure 7. Photographs of the billboard targets made 
with the long-focus camera. 


line which, when extrapolated to zero range, passes 
through a contrast of approximately minus 1. The 
atmospheric attenuation coefficient was computed 
from the slope of this line. The meteorological range 
(Section 2.2.5) that corresponds to this value of 
is 47.3 miles. 



Figure 8. Plot showing the variation with distance of 
the apparent contrast of the billboard targets as shown 
in Figure 7. 

The same contrast scale has been used for both negative and 
positive contrasts. From the slope of the line drawn through the 
black target points, P = 0.0470 per thousand yards. 

Time: 10:00 a.m. to 11:00 a.m. 

Sky: Clear, light smoke over Connecticut 
Estimated visibility: 50 miles 
Atmospheric pressure: 1014.6 millibars 
Temperature: 57 degrees F. 

Dew point: 47 degrees F. 

Relative humidity: 69 per cent 
Wind: NW 6 miles per hour 


The points representing the contrast of the white 
targets cannot be fitted by a straight line. Random 
scattering of white target points was observed in 
most of the experiments, but no systematic trend 
was noted. The scattering of these points appears 
to result from differences in the lighting of the tar¬ 
gets. Even on seemingly clear days traces of cloud 


CONFIDENTIAL 











THE VISIBILITY OF OBJECTS VIEWED ALONG A HORIZONTAL PATH 


25 


formation at very high altitudes often cause the 
illumination over the surface of the earth to vary 
slightly, and these variations cause considerable un¬ 
certainty in the inherent contrast of a white target 
against a background of horizon sky. Most targets 
of naval or military interest are of low reflectance, 



Figure 9. Plot showing variation with distance of the 
contrast of the billboard targets on a day when the 
Connecticut shore (12 miles distant) was not visible. 

From the slope of the line representing the black targets 
P = 0.209 per thousand yards. 

Time: 10:00 a.m. to 11:00 a.m. 

Sky: 7/10 cirrus H 6 
Ceiling: 25,000 feet 

Estimated visibility: Between 3 and 12 miles 
Atmospheric pressure: 1011.9 millibars. 

Temperature: 74 degrees F. 

Dew point: 65 degrees F. 

Relative humidity: 74 per cent 
Wind: NW light 

so that small variations in illumination do not 
greatly alter their inherent contrast. In the limiting 
case of a completely black target, illumination dif¬ 
ferences have no effect. For this reason, meteorolo¬ 
gists specify black targets for use in estimating the 
daylight visual range (Section 2.2.5). 

These same effects are to be noted in Figure 9, 


which represents a corresponding experiment con¬ 
ducted on a hazy day when the visibility was 
greater than three and less than twelve miles. The 
meteorological range computed from the value of /$ 
was 10.6 miles for the black targets. Little relia¬ 
bility can be placed upon the points representing 
the white targets, but the trend of the data indi¬ 
cates the general validity of the principle that the 
apparent contrast of all targets changes exponen¬ 
tially with distance. 

The results obtained with the photographic tele¬ 
photometer were duplicated, but not improved, by 
experiments conducted with several othe'r types of 
telephotometers housed in a small temporary lab¬ 
oratory building (Figure 10) on the shore of Cold 



Figure 10. This temporary building on the shore of 
Cold Spring Harbor was used to house the telepho¬ 
tometer. 

Telescope can be seen protruding from building under rain 
shield <left front). Building was called “Celotex Lodge.” 

Spring Harbor. The design of a telephotometer 
suitable for measuring the apparent contrast of the 
targets is not easy, if the size of the most distant 
board is held within practical limits. A telescope 
objective, 6 inches in diameter and 4 meters in focal 
length, was used to form an image of the targets. 
Careful external and internal baffling was used in 
the telescope, and the stray light in the system was 
found to be exceedingly small. During preliminary 
experiments, the photometric measurements were 
made with a Macbeth illuminometer mounted on 
the telescope to form a Maxwellian view device. 
The precision of such a telephotometer cannot be 
made high because of the small relative aperture of 
the objective. Subsequently, a Photovolt electronic 
photometer was used. A satisfactory compromise 


CONFIDENTIAL 








26 


THE SCREENING OF TARGETS BY THE ATMOSPHERE 


between sensitivity and stability could not be found, 
and finally the a-c photoelectric telephotometer 
shown in Figure 11 was built. 80 



Figure 11. Photograph of the a-c telephotometer 
used by the Tiffany Foundation for measuring the 
apparent brightness of the billboard targets. This in¬ 
strument was housed in Celotex Lodge (Figure 10). 

2 ’ 2,4 Two Misconceptions 

Two apparenty nonexistent “effects” are often 
mentioned in the literature of meteorology. These 
are known as the ground-glass plate effect 12a and 
the edge effect ; 12b they refer to loss of sharp detail 
by low-angle scattering and diffraction of light 
around the target respectively. Neither is based 
upon sound theoretical reasoning, and neither has 
been demonstrated experimentally. Both effects are 
the result of attempts to explain certain visual im¬ 
pressions in terms of the scattering properties of the 
atmosphere. Actually, these illusions are natural 
consequences of the mechanism of human vision, 
and are dealt with properly in the next three chap¬ 
ters of this volume. 

The ground-glass plate effect was explored pho¬ 
tographically with the long-focus camera (Figure 
6) at the Tiffany Foundation. Photographs of the 
resolving power targets shown on the left in Figure 
5 (top) were photographed in both clear and foggy 
weather. When the contrast (gamma) of the photo¬ 
graph was made equal to e& x as determined from 
transmissometer readings, the targets were resolved 
equally well in all photographs. The experiment w r as 
repeated using natural objects as targets, and the 
same conclusion was reached. No fine details were 
obliterated by the haze. The meteorologist may 
safely consider that the ground-glass plate effect 
does not exist. 

The edge effect was explored with the a-c photo¬ 
electric telephotometer (Figure 11) at the Tiffany 


Foundation. This instrument was used to compare 
the apparent brightness of several black targets 
visible against a background of horizon sky. The 
angular size of the targets ranged from 0.8 minute 
to more than 1 degree. No difference in the ap¬ 
parent brightness of the targets was found. This 
conclusion is supported by the electromagnetic 
theory of light. Inasmuch as all the targets are large 
compared with the wavelength of light, diffraction 
around them wrnuld not be expected to increase their 
apparent brightness. The meteorologist may safely 
consider that the edge effect does not exist. 

2 2 5 Meteorological Range 

The optical effect of the atmosphere is usually 
reported by meteorologists in terms of the daylight 
visual range or visibility. By international agree¬ 
ment, the daylight visual range is the distance at 
which a large dark object on the horizon is just rec¬ 
ognizable against the sky background. The rela¬ 
tionship between the daylight visual range and 
has not yet been established by international agree¬ 
ment, but it is standard practice at the Naval Re¬ 
search Laboratory and elsewhere to assume that an 
object subtending a large angle at the eye can be 
recognized in the daytime when its brightness differs 
from that of its sky background by as much as 2 
per cent. The contrast of a black target is by defini¬ 
tion — 1. If —1 is substituted for C 0 , and —0.02 for 
C x equation (14) can be written 

ln 0S2 = ^ = 3 ' 912 ’ (16) 

where A" has been replaced by the symbol v. The 
distance v will henceforth be referred to as the 
meteorological range. It is, by definition, that hori¬ 
zontal distance for which the transmittance of the 
atmosphere e~& v is 2 per cent. 

It must be borne in mind that the daylight visual 
range refers to the distance at which large black 
objects can just be recognized against a bright 
daytime sky. By a large object is meant an object 
so large that the angle it subtends at the eye of the 
observer is sufficiently great so that a greater angle 
would not increase the reported value of daylight 
visual range. The visibility marks available to a 
practicing meteorologist are rarely of sufficient an¬ 
gular size. In other words, the daylight visual range 
is seldom so short that a large object, such as a 


CONFIDENTIAL 






THE VISIBILITY OF OBJECTS VIEWED ALONG A HORIZONTAL PATH 


27 


house or a tree, subtends a sufficiently large visual 
angle. As a result, the visibility reported by meteor¬ 
ologists is usually somewhat less than the meteor¬ 
ological range defined by equation (16). A Civil 
Aeronautics Administration report 253 presents re¬ 
sults which indicate that visibility as ordinarily re¬ 
ported averages to be three-quarters of the meteor¬ 
ological range. 

Under homogeneous lighting conditions, the 
meteorological range is the same in all directions. 
This is implied by equation (16), inasmuch as v 
depends only on the nondirectional quantity /?. The 
extent to which the apparent brightness of a distant 
object differs from its inherent brightness depends 
upon the bearing of the object relative to the sun, 
as shown by equation (4). This equation involves 
the scattering coefficient o, the magnitude of which 
depends upon the direction in which the scattering 
takes place. However, the variation of apparent con¬ 
trast with distance is independent of o , as shown by 
equation (14). This is a valuable consequence of the 
definition of contrast. 


Directional Variation of the 
Daylight Visual Range 


s 


K 




(17) 


where K is a constant. Combining equations (6) and 
(17) 


KB h 

oq ’ 


(18) 


Let directions 1 and 2 be denoted by subscripts. 
Then, 

KB i 
oi q 1 


and 


By division 


Sl 


s 2 — 


KB 2 
02 q ' 

B\ o 2 
B 2 cq 


(19) 


But under homogeneous lighting conditions Si = s 2 . 
Hence, 


Bi __ 0i 
B 2 02 


( 20 ) 


Assuming equation (19) to apply under the non- 
homogeneous lighting condition, then by the substi¬ 
tution of equation (20) in equation (19) 


51 B\ B 2 

5 2 B 2 Bi' 


( 21 ) 


Under nonhomogeneous lighting conditions the 
daylight visual range may not be the same in all 
directions. The variation may be caused by banks 
of clouds or smoke on the horizon; or it may be due 
to inhomogeneities in the atmosphere along the line 
of sight. The latter may be caused by temperature 
variations, such as those encountered when the line 
of sight passes over both land and water, or by 
local banks of haze or fog. There may be local 
variations in the luminous density due to differing 
reflectivities of the natural terrains along the line 
of sight, or to cloud shadows. 

When the daylight visual range is to be predicted 
from values of measured by an instrument such 
as the Bureau of Standards transmissometer, it may 
be necessary to allow for directional variations. 
When interest in this problem was expressed by the 
Navy Bureau of Aeronautics, the following possible 
procedure for making such allowances was sug¬ 
gested. 

Let it be assumed from equation (16) and the 
subsequent discussion that the daylight visual range 
s is inversely proportional to /?, 


where the primed quantities refer to normal homo¬ 
geneous conditions. 

In order to measure the quantities involved in 
equation (21), a horizon-scanning photometer was 
built by the research laboratories of the Interchem¬ 
ical Corporation under contract OEMsr-697. 8d This 
instrument, shown in Figures 12 and 13, was devel¬ 
oped to permit the brightness of a 4-degree zone 
above the horizon to be measured in any desired 
direction. Values of B 2 /R/ can be obtained with 
this instrument, and expressed in the form of polar 
curves, from which the ratio Si/s 2 for any existing 
nonhomogeneous conditions could be computed by 
means of equation (21) from measured values of 
Bi/B 2 . 

For example, consider Figure 14, which shows a 
polar plot of horizon brightness as measured by the 
horizon-scanning photometer. The dotted curve in¬ 
dicates the brightness of the horizon sky under ho¬ 
mogeneous conditions. The solid curve represents an 
occasion when the northeast horizon appeared ab¬ 
normally bright. Let it be required to predict the 
daylight visual range at an azimuth of 40 degrees 


CONFIDENTIAL 




28 


THE SCREENING OF TARGETS BY THE ATMOSPHERE 



Figure 12. Photograph of horizon scanning photom¬ 
eter (rear view). 

The diffusing globe mounted on the top of the photometer 
was used to measure the luminous density ( q ). 



Figure 13. Schematic diagram of the horizon scan¬ 
ning photometer. 

Diffusing globe (1) and photocell (2) were used to measure 
the luminous density (g). Lens (3) imaged the sky just above 
the horizon on stop (7). A photocell (4) received the light which 
passed through the stop. A sunshade (5) and baffles (6) reduced 
stray light. 


(measured clockwise from the north) by means of 
equation (21). Let this direction (shown by the 
dashed line in Figure 14) be referred to as direc¬ 
tion 1, and let direction 2 be any direction for which 
the solid curve has normal shape. Since, in this case, 
the normal and abnormal curves coincide except in 
the northeast, equation (21) becomes 

_ 

Si — s 2 . 

Substituting values from Figure 14, 


_ 7,660 
Sl ~ 3,660 


22 = 47 miles. 


At the request of the Navy, the horizon-scanning 
photometer was turned over to the Aircraft Cam¬ 
ouflage Sub-Section, Tactical Test, Naval Air 
Station, Patuxent River, Maryland. No experi¬ 
mental test of equation (21) is known to have been 
made. 


2 2,7 Backgrounds Other Than 

the Horizon Sky 

Throughout the foregoing discussion, the target 
has been assumed to be viewed against a back¬ 
ground of horizon sky. Under some circumstances, 
the target may be seen against other backgrounds. 


N 

0® 



Figure 14. Polar plot of the brightness of the horizon 
sky measured by the horizon scanning photometer at 
the Naval Air Station, Patuxent River, Maryland. 

Sun ratio: 13.4; sun elevation: 28 degrees; visibility: 22 miles. 
The broken curved line indicates the brightness of the horizon 
sky under corresponding homogeneous atmospheric conditions. 


For convenience, consideration of the apparent con¬ 
trast of a target under such circumstances will be 
deferred until the visibility of objects viewed down¬ 
ward along slant paths has been discussed (Section 
2.3.7). 


CONFIDENTIAL 
























THE VISIBILITY OF OBJECTS DOWN A SLANT PATH 


29 


23 THE VISIBILITY OF OBJECTS 
VIEWED DOWNWARD ALONG A 
SLANT PATH 

An aerial observer views any object on the ground 
along a slant path throughout which the scattering 
coefficients ft and o vary with altitude. If the strati¬ 
fication of the atmosphere is continuous, these coeffi¬ 
cients vary regularly and in a readily predictable 
manner, but such a condition is rare. Usually the 
atmosphere is composed of optically dissimilar 
strata, the boundaries of which are often sharply de¬ 
fined and within which /? and a vary with altitude. 
A method for solving practical visibility problems 
involving any set of atmospheric conditions which 
may exist is described in Chapter 5. However, the 
case of an atmosphere having continuous, regular 
optical stratification must be discussed first in order 
to provide a basis for treating the case of discon¬ 
tinuous stratification. 

2.3.i The Differential Equation 

Along slant paths, the fundamental scattering- 
processes are the same as along horizontal paths of 
sight. The differential equation corresponding to 
equation (1) is 

+ ( 22 ) 

where the meaning of the symbols is shown by 
Figure 15. The subscript y is used to indicate that 
the scattering coefficients /3 and a and the luminous 
density q are functions of the altitude coordinate y. 

An attempt was made to use the spectrogeograph 
(Section 5.4.1) to explore the variation of luminous 
density with altitude, but no variation was detected 
up to the highest altitude attained (15,000 feet). 
A similar lack of variation was found on other 
occasions when the illumination on a horizontal 
plane was measured at altitudes up to 18,000 feet. 
During all these flights, the solar altitude was in the 
neighborhood of 50 degrees. From these experiments, 
it is believed that ordinarily the variation of q with 
altitude is insignificant. 15 If q is regarded as a con- 

b This approximation cannot always be made. For exam¬ 
ple, just after sunset the luminous density is much greater 
at high altitudes than it is near the ground. The exact 
range of solar altitudes within which the variation of q 
with altitude may be neglected is not known. Any extension 
of the research described in this volume should include an 
investigation of this matter. 


stant, equation (22) can be solved by direct inte¬ 
gration, provided a simple functional relationship 
exists between and y and between o y and y. 



The Standard Atmosphere 

Meteorologists sometimes refer to a standard at¬ 
mosphere in which the temperature-lapse rate within 


Table 1 


Altitude 

(feet) 

Pressure 
of mercury 
(inches) 

Altitude 

(feet) 

Pressure 
of mercury 
(inches) 

- 1.000 

31.02 

14,000 

17.58 

0 

29.92 

15,000 

16.89 

1,000 

28.86 

16,000 

16.22 

2,000 

27.82 

17,000 

15.57 

3,000 

26.82 

18,000 

14.94 

4,000 

25.84 

19,000 

14.34 

5,000 

24.90 

20,000 

13.75 

6,000 

23.98 

22,000 

12.64 

7,000 

23.09 

24,000 

11.60 

8,000 

22.22 

26,000 

10.63 

9,000 

21.39 

28,000 

9.73 

10,000 

20.58 

30,000 

8.89 

11,000 

19.79 

35,000 

7.04 

12,000 

19.03 

40,000 

5.54 

13,000 

18.29 

50,000 

3.42 


the troposphere is 6 F per thousand feet of altitude. 
The variation of pressure with altitude is given in 
Table l. 27 


CONFIDENTIAL 












30 


THE SCREENING OF TARGETS BY THE ATMOSPHERE 


From the standpoint of visibility, the refracting, 
scattering, and absorbing properties of the atmos¬ 
phere determine its effectiveness in screening targets. 
It is beyond the scope of this report to discuss the 
deleterious effect on image quality sometimes pro¬ 
duced by local variations in the refractive index of 
the air along the line of sight, but it is a well-known 
physical principle that the refractive index of a gas 
is proportional to its density. Similarly, the absorb¬ 
ing and scattering properties of a gas are propor¬ 
tional to its density, that is, to the number of 
molecules per unit volume. Therefore, the data 
shown in Table 1 have been converted into the rela¬ 
tive number of molecules per unit volume, account 
having been taken of the effect of the temperature- 
lapse rate by means of the equation of state of a 
perfect gas. The result is shown in Table 2. 


Table 2 


Altitude 

(feet) 

Relative number 
of molecules 
per unit volume 

0 

1.000 

1,000 

0.956 

2.000 

0.918 

3,000 

0.878 

4,000 

0.841 

5.000 

0.804 

6.000 

0.770 

7,000 

0.736 

8.000 

0.703 

9,000 

0 672 

10.000 

0.642 

12,000 

0.586 

14,000 

0.534 

16,000 

0.485 

18.000 

0.440 

20,000 

0.399 

22,000 

0.361 

24,000 

0.326 

28.000 

0.295 

28,000 

0.288 

30,000 

0.233 


The atmosphere may contain, besides air. micro¬ 
scopic water droplets, dust, rain, snow, smoke, 
etc. Water particles and dust are usually homo¬ 
geneously distributed except for stratification. Let 
the optical standard atmosphere be defined as a 
homogeneous atmosphere in which the water parti¬ 
cles, dust particles, and air molecules are subject 
to continuous vertical stratification at the rate im¬ 
plied by Table 2. 

An analytic expression for the data in Table 2 is 
required before differential equation (22) can be 



Figure 16. Variation with altitude of relative density 
of scattering particles. 

Points represent optical standard atmosphere defined by 
Table 2. These data have been approximated by the straight 
line. Slope:. 2/100. 


solved. Figure 16 shows a semilogarithmic plot of 
the relative number of particles per unit volume as 
a function of altitude. Within the range of altitude 
represented, the data may be approximated by a 
straight line, the equation of which is 

hi — g-»/21,700 (23) 

Ao 

Accordingly, let 

p y = p 0 e~ y/21 > 700 and o„ = o 0 e~ y l 21 ’™>, (24) 

where y is to be expressed in feet. Along any slant 
path making an angle 0 with the horizontal 

y — r sin 6. (25) 

After combination with equations (24) and (25), 
equation (22) becomes: 

~ = — pote" *'" e, - 1 . T,!0 + a 0 qe- r ,,n e,21 ' 70 °. (26) 


Optical Slant Range 

Equation (26) can now be integrated directly: 


dt 


o 0 q — Pot 



(27) 


CONFIDENTIAL 










THE VISIBILITY OF OBJECTS DOWN A SLANT PATH 


31 


whence 

dog — PBr _ gjj 

a 0 q-l5B 0 - e ’ 

where 


R = 


21,700 
sin 0 


e ~R sin 0/21,700 


]■ 


(28) 

(29) 


The quantity R will henceforth be referred to as 
the optical slant range. Physically, it is the path 
length within a homogeneous atmosphere, having 
no lapse rate or pressure gradient, along which light 
would encounter the same number of particles ac¬ 
tually encountered along the path of length R 
within the standard atmosphere. In other words, R 
is that horizontal distance which contains as much 
air as does the slant distance R. 


2,3,4 Variation of Apparent Brightness 
Along Slant Paths 

Equation (28) can be written in a form similar to 
equation (4): 

+ (30) 

For the special case of 0 = 0, R = R = X, and 
equation (30) reduces to equation (4). The factor 


o 0 <7 _ oq _ R 


By extension, when 6 — 0 the factor 


(31) 



(32) 


where B H ' is the brightness of the horizon sky in the 
particular directions indicated by the arrows m and 
n in Figure 17. In these directions, the brightness of 
the horizon is determined by space light scattered 
at the same angle from the rays of the sun as the 
space light scattered in the direction of the aerial 
observer. Under most circumstances, directions m 
and n can be found and B H ' measured directly. How¬ 
ever, when the observer is nearly “in the sun” as 
seen from the target, the cone produced by rotating 
his line of sight about the direction of the rays of 
the sun will not intersect the horizontal plane. In 
this case, resort must be had to special photometric 
equipment for determining Ooq/P 0 - 


Optical Equilibrium 

Equation (26) indicates that when fi 0 t = c 0 q, 
dt/dr = 0. Thus, a condition of optical equilibrium 
(Section 2.2.1) exists along slant paths in the sense 
that any object whose inherent brightness equals 
0 oQ//?o appears equally bright when viewed from 
any distance. Equation (32) indicates that objects 
having the brightness of the horizon sky in the m 
and n directions (Figure 17) fulfill the conditions 



Figure 17. Arrows m and n indicate the directions in 
which the brightness of the horizon sky is determined 
by light scattered from the rays of the sun at the same 
angle as light scattered toward the aerial observer. 

for equilibrium. The apparent brightness of darker 
objects increases with distance, while that of brighter 
objects decreases. In the limit, the apparent bright¬ 
ness of all objects on the ground approaches the 
equilibrium value asymptotically. Thus, when an 
observer aloft cannot see the ground because of 
haze, the apparent brightness of the earth is the 
same as the brightness of the horizon sky seen by 
an observer on the ground looking in the morn di¬ 
rections. A measurement of the apparent brightness 
of the earth by such an aerial observer may be taken 
as the value of Ooq/fio- 

The Attenuation of Brightness Differences 
An important consequence of equations (4) and 
(30) is the theorem that along either slant paths or 
horizontal paths brightness differences are exponen¬ 
tially attenuated. That is to say, if the target and its 
background have inherent brightnesses B 0 and B 0 ' 


CONFIDENTIAL 










32 


THE SCREENING OF TARGETS RY THE ATMOSPHERE 


respectively, so that the difference in their bright¬ 
nesses A B 0 = B 0 — B 0 ', then from equation (30) 
their apparent brightness difference at range R is 
given by 

A B r = A B 0 e~& (33) 

2.3.5 Y ar i a tion of Apparent Contrast 
Along Slant Paths 

The variation of apparent contrast along slant 
paths is more complex than along horizontal paths. 
This complexity arises because the apparent bright¬ 
ness of the background is a function of R. Let the 
inherent contrast between the target and its back¬ 
ground be defined by 


and the apparent contrast at range R by 

C R = ^ . (35) 

Dr 

If equations (30), (33), and (34) are substituted 
in equation (35), the law of contrast attenuation 
along slant paths is found to be 



The Sky-Ground Ratio 

The quantity B H '/Bo has been called the sky- 
ground ratio. On a uniformly overcast day when 
the earth is covered with snow, B H ' == B 0 , and the 
sky-ground ratio is unity. Equation (36) then re¬ 
duces to 

C R = C 0 e~ B ** (37) 

along any line of sight, vertical, slant, or horizontal. 
Under other circumstances, the sky-ground ratio 
provides a means by which the law of contrast 
attenuation along slant paths can be adjusted for 
the effect of lighting conditions, ground reflectance, 
and the orientation of the line of sight with respect 
to the sun. 

Typical values of the sky-ground ratio for a slant 
path such as that shown in Figure 15 are given in 
Table 3. 


Table 3 


Sky 

condition 

Ground 

condition 

Sky-Ground 

ratio 

Overcast 

Fresh snow 

1 

Overcast 

Desert 

7 

Overcast 

Forest 

25 

Clear 

Fresh snow 

0.2 

Clear 

Desert 

1.4 

Clear 

Forest 

5 


2 3 7 Horizontal Sight Paths 

Along the horizontal paths of sight discussed 
earlier in this chapter, the background of the target 
was assumed to be the horizon sky. In such a case 
Bo and B H ' are identical, and therefore, since R = 
R=X, equation (36) reduces to equation (14). 
However, under some circumstances, the target may 
be viewed against a background other than the sky. 
For example, a ship at sea may be seen against a 
background formed by a distant land mass, or the 
“target” may be a numeral painted on the side of a 
ship. In such a case, the apparent contrast can be 
calculated by means of equation (36), if the sky- 
ground ratio is replaced by the ratio of the bright¬ 
ness of the horizon sky in the direction of observa¬ 
tion to the inherent brightness of the background 
of the target. 

The Visibility of Military and 
Naval Targets 

Targets of military and naval interest ordinarily 
subtend a very small angle at the eye of the ob¬ 
server when they are viewed at limiting range. In 
such cases, the limiting range of visibility is gov¬ 
erned not only by the condition of the atmosphere, 
but also by the angular size and effective contrast of 
the target. The following chapter presents data on 
the perceptual capacity of the human observer un¬ 
der virtually all circumstances encountered out¬ 
doors. The manner in which this information can be 
combined with the laws of contrast attenuation by 
the atmosphere will be treated in Chapter 4 for the 
case of targets viewed along a horizontal path, and 
in Chapter 5 for the case of targets viewed down¬ 
ward along a slant path. 


CONFIDENTIAL 








Chapter 3 

PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


3i INTRODUCTION 

T he visibility of targets is influenced not only by 
such physical factors as were discussed in the 
preceding chapter, but also by certain physiological 
factors. These include the effective brightness and 
color contrasts of the target against its background, 
the size and shape of the target, the brightness level 
to which the eyes of the observer are adapted, and 
the conditions and technique of observing. 

Previous investigations have seldom included the 
entire useful range of any one of these factors and 
have never undertaken to include the combined ef¬ 
fect of all of them. Consequently, existing experi¬ 
mental data have rarely been applicable to the 
visibility problems encountered in naval and mili¬ 
tary operations. 

From the outset of the visibility program con¬ 
ducted under the supervision of Section 16.3, NDRC, 
it was the basic plan to investigate the various fac¬ 
tors one by one over their entire ranges in such a 
manner that the final data could be used to predict 
the visibility of naval and military targets. The 
investigation of the physiological factors was di¬ 
vided into two basic programs, one concerned with 
the influence of brightness contrast 23a > b and the 
other with the influence of color contrast. 

The Louis Comfort Tiffany Foundation, of Oyster 
Bay, New York, was requested under Contract 
OEMsr-597, to conduct an extensive investigation 
of the influence on visibility of the brightness con¬ 
trast of targets of all sizes and shapes, at all bright¬ 
ness levels encountered outdoors, day or night. This 
research is described in OSRD Report No. 6401, 
entitled, Visibility of Targets 24 

The influence of color contrast was investigated 
by the Eastman Kodak Company, Rochester, New 
York, under Contract OEMsr-1070, and the results 
are to be found in OSRD Report No. 4541, Influence 
of Color Contrast on Visual Acuity. 22 As shown 
therein, it is possible to evaluate any color contrast 
in terms of a brightness contrast yielding the same 
visual acuity. This makes it possible to combine the 
influences of color contrast and brightness contrast 
in such a way that the visual capacities of a typ¬ 
ical human observer can be expressed quantita¬ 
tively. 


3 2 INFLUENCE OF BRIGHTNESS 
CONTRAST ON VISIBILITY 

The key to a method for investigating the in¬ 
fluence of brightness contrast on the visibility of 
targets was given by a preliminary experiment in 
which the visibility of the silhouettes of typical 
naval vessels and aircraft was compared with the 
visibility of circular spots. This experiment indi¬ 
cated that, ordinarily, uniform targets of equal area 
and equal apparent contrast are equally visible re¬ 
gardless of their shape. 3 Accordingly, a fundamental 
investigation of the visibility of circular targets was 
first undertaken. 

3 21 The Visibility of Circular Targets 

The major portion of the Tiffany investigation 
was devoted to the determination of the contrast of 
circular targets of selected diameters which were 
just visible against uniform backgrounds having 
various brightnesses from 10~ 6 to 100 foot-lamberts. 
The target diameters subtended angles from 0.6 to 
400 minutes at the eyes of the observers; and tar¬ 
gets both brighter and darker than their back¬ 
grounds were used. 

Preliminary Experiments 

The Tiffany Foundation was forewarned by the 
published results of earlier investigators to expect 
the reproducibility of visual experiments to be 
highly erratic. For this reason, their first experi¬ 
ments were designed to yield results of the highest 
possible precision in order to determine the general 
nature of visual functions. 

The Eight-Position Method. A circular spot of a 
given size was produced by projection at any one of 
eight equally spaced positions around a clearly 
visible orientation spot located at the center of the 
screen. The circumference of the spot track thus 
formed was such that a straight line of equal length 
would have subtended an angle of approximately 15 
degrees at the eyes of the observers. The target was 
projected for 6 seconds, during which time the ob¬ 
servers were notified by the sound of the buzzer that 

a See Section 3.2.10 for a quantitative discussion of the 
effect of target shape. 


CONFIDENTIAL 


33 



34 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


the target was present. The 6-second searching time 
was considered to correspond to a scanning rate of 
2.5 degrees per second, a value consistent with the 
reported practice of lookouts aboard German sub¬ 
marines. It was sufficiently short that, with ten 
observers, a large enough number of observations 
could be taken to permit the liminal contrast of the 
target to be determined with a high degree of relia¬ 
bility. Ordinarily, a total of 2,880 observations were 
made in determining each datum point. 

Final Experiments 

Later in the war, after the subject of search had 
become a major interest of the Operations Research 
Group, COMINCH, U. S. Navy, it was no longer 
necessary or desirable to make any assumption con¬ 
cerning the rate of search to be employed by an 
observer in the field. Moreover, it was found that 
no simple relation exists between the diameter of the 
spot track and the effect of the time allowed for 
search. For these reasons it is felt that the results of 
the preliminary experiments are of little practical 
importance other than to illustrate the general shape 
of the visibility curves and to demonstrate that 
highly reproducible visual experiments are possible. 

The Single-Position Method. The final experi¬ 
ments were designed to determine the upper limit 
of visibility (lower limit of just-visible contrast), 
the time for observation being such that a longer 
period produced no lower value of liminal contrast. 


For targets of low contrast seen at high levels of 
adaptation brightness, the time required to attain 
ultimate scores was found to be impracticably long. 
When, however, the target was confined to a single 
fixed position and the observer was required to re¬ 
port only whether or not the target was visible, 
maximum scores could be obtained with reasonably 
short observation times. The target was frequently 
absent, so that mere guessing was discouraged and 
allowance for its influence could be based on the 
erroneous reports of the presence of the target. 

The results of the preliminary 8-position experi¬ 
ments served as a valuable guide in selecting the key 
points required to produce the final curves. This 
was fortunate, since the single-position experiments 
could not be conducted as rapidly as had the 8-posi¬ 
tion ones. Most of the experiments with large targets 
at high-brightness levels were repeated with the 
single-position method, and a representative selec¬ 
tion of experiments for smaller targets and lower 
brightnesses was also repeated. The precision of the 
results obtained with the single-pqsition method was 
inferior to that of the 8-position method. Conse¬ 
quently, the curves representing the single-position 
results were drawn wdth spacings and slopes similar 
to those of the curves representing the results of the 
8-position experiments. 

The equality of visibility for equal light and dark 
contrasts was demonstrated by experiments with the 
8-position method. The influence of target shape on 



Figure 1 . Observation room at Tiffany Foundation. 


CONFIDENTIAL 










INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


35 



Figure 2. Control and recording chamber. 


visibility was determined for various brightnesses, 
contrasts, and target shapes by the single-position 
method. 

322 Apparatus 

Certain functional requirements governed the 
arrangements of the apparatus used by Tiffany. 
These were: 

1. Simultaneous but independent observation by 
as many as 10 observers. 

2. Presentation of the target at frequent and regu¬ 
lar intervals. 

3. Use of the psychophysical method of constant 
stimulus. 

4. Photometric measurements based exclusively 
on the use of standard lamps and the inverse square 
law. 

Observation Room 

The observations were made in a room 63 feet 
long, 12 feet wide, and 10 feet high. This room, a 
sketch of which is shown in Figure 1, was con¬ 
structed of plywood panels inside a room at the Tif¬ 


fany Foundation. A large control and recording 
chamber (Figure 2) was situated at one end of the 
observation room. Ten upholstered theatre seats 
were located just inside the observation room, five 
on and five under a balcony which extended across 
the control-room end of the observation room, as 
shown in Figure 3. 

The floors, walls, ceilings, and all the furnishings 
and accessories within the observation room had flat 
white finishes. The side panels of the room, each 10 
feet square, were arranged as louvers, opposite pairs 
converging toward the front of the room, as shown 
in Figures 4a and 4b. The front wall, 10 feet square, 
was smooth and unobstructed and had, during most 
of the experiments, only one small hole in the center 
through which light for the fixation spot was ad¬ 
mitted. About 6 feet were available beyond the 
front wall of the observation room and this space 
was occupied by apparatus during experiments with 
very small targets. 

Illumination Arrangements 

Hidden from the observers, in troffers behind the 
front edges of the side panels were five banks of 


CONFIDENTIAL 




























36 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 



Figure 3. Photograph of observers in stations, and projection lens. 


lamps on each side of the room (see Figure 4b). 
These lamps illuminated the room quite uniformly, 
especially the front wall, which served as the field 
of observation (see Figure 5). Part or all of the 
lamps in each troffer could be operated, in order to 
produce several levels of illumination in the room 
and on the observation screen. For moderate and 
high levels, general service bulbs of various wattages 
were installed as shown in Figure 6. 

In all cases, the arrangement of lamps was in¬ 
tended to produce a gradual gradation of brightness 
from a maximum at the screen to approximately 10 
per cent of the maximum near the observers. A tele¬ 
photometer was used to insure that the desired pat¬ 
tern of brightness was attained and maintained 
throughout the program. For experiments with tar¬ 
gets brighter than their background, the brightness 
relative to that of the screen was approximately 95 
per cent for the first panel, 75 to 85 per cent for the 
second panel, 60 to 70 per cent for the third panel, 


45 to 50 per cent for the fourth panel, 25 to 35 per 
cent for the fifth panel, and 8 to 15 per cent for the 
sixth panel. 

To achieve low levels of illumination, small bulbs 
were placed inside light-tight brass tubes, each con¬ 
taining several plates of ground glass through which 
the light had to pass. (See Figure 7.) The luminous 
output of these units was adjustable by varying the 
distance from the bulbs to the first ground glass, by 
varying the number and separation of the plates, 
and by placing opaque annular diaphragms between 
the bulb and the first ground glass. These dia¬ 
phragms had holes of various diameters, to reduce 
the amount of light incident upon and transmitted 
by the layers of ground glass. 

For the observation of targets darker than their 
background, part or all of the illumination of the 
surrounding screen was by projection, as described 
in Section 3.2.3. In the majority of these experi¬ 
ments, the gradation of brightness from the front to 


CONFIDENTIAL 











INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


37 



STANDARD 

LAMP 

HOUSING 


STORAGE ELECTRICAL MAIN TARGET 



APPARATUS 

Figure 4. Top view elevation of observation room; bottom view plan of observation room. 



Figure 5. Photograph of observation room from 
observers’ seats. Note circular target at center of 
screen. 

the rear panels was similar to that for experiments 
with targets brighter than their background. 

323 Projection Equipment 

All the targets were produced by projection of 
additional light on the screen. A single lantern-slide 
projector (Figure 8) was used for targets brighter 
than their surroundings (Arrangements I to IV). 


Additional projectors were used in Arrangements V 
and VI for targets darker than their backgrounds. 
Arrangement VII was used to project targets in a 
single position on the center of the screen. 

Arrangement I. This was used for experiments 
with highest background brightness when the great- 



Figure 6. Cross section of troffers of observation 
room, with lamp arranged for moderate and high 
brightness experiments. 


est projector output was required. Several inter¬ 
changeable metal plates with single round holes of 
various diameters (in the focal plane of the projec- 


CONFIDENTIAL 




















































































38 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


tor) produced targets subtending 3.60, 9.68, 18.2, 
55.2, and 121 minutes at the eyes of the observers. 
An achromatic prism larger than the aperture, with 
an angle of about 4.5 degrees, was mounted in front 



Figure 7. Cross section of troffer and lighting unit fol¬ 
low brightness experiments. 


of the projector lens. By rotating this prism (Figure 
9), the target could be made to appear at any one 
of eight equiangular positions, each 32 inches from 
the center of the observation screen. A set of eight 
electric contacts was provided, one of which com¬ 
pleted a circuit to indicate the position of the target. 

The contrast of the projected bright target was 
governed by filters placed between the condenser 
lens and the target aperture, as shown in Figure 8. 
Four filters mounted in a circular disk (Figure 10) 
could be interposed, one at a time, to achieve trans¬ 


missions of 1.000 (empty aperture), 0.762, 0.540, 
0.372, and 0.235. The location of the filter disk, and 
consequently the contrast, was indicated electrically 
by an 8-point switch, shown in Figure 10. An opaque 
plate (Figure 11) served as a shutter to prevent the 
projection of the image while its position and con¬ 
trast were being changed. An electric contact closed 
a circuit so that a buzzer sounded whenever the 
shutter was open. The duration of each presentation 
of the target was 6 seconds, with a 6-second interval 
between. 

Arrangement II. This was used for experiments at 
intermediate brightnesses (Figure 12). The maxi¬ 
mum contrast was governed by neutral absorbing 
filters inserted between the condenser and the con¬ 
trast-filter disk in Arrangement I. These range 
filters were selected for each experiment according 
to the results of a preliminary series of observations. 
Once selected, they were unchanged throughout each 
experiment. The spectral transmittances of typical 
range filters are shown in Figure 13. 

Arrangement III. This w r as used in one experiment 
with small targets on the highest background bright¬ 
ness, for which additional light was provided by a 
spherical mirror placed behind the projection lamp 
in Arrangement II (Figure 14). 

Arrangement IV. This was used in all experiments 
with low levels of adaptation and targets brighter 
than their backgrounds. This arrangement (Figure 
15) was the same as II, with the addition of a series 
of ground-glass plates one-half inch from the target 


PROJECTION 

LAMP 

\ 


—i— 

LJ 


WATER 

CELL 


\ 



CONDENSER 

LENSES 


APERTURE 



CONTRAST 

FILTERS 


HAND- 

OPERATED 

SHUTTER 



PROJECTION 

LENS 



ROTATING 

PRISM 


Figure 8. 


Projector Arrangement I, for targets lighter than background, with highest background brightness. 


CONFIDENTIAL 











































INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


39 


aperture on the side toward the condenser lens. This 
diffusing glass reduced nonselectively the flux pro¬ 
jected by the system and will be called the second- 




SPROCKET GEAR 
POSITIONING HANDLE 
POSITIONING HOLE 

CHAIN 

THIN PRISM 


Figure 9. Mechanism for rotating prism. 



Figure 10. Control filter disk with indicating switch. 


ary source of the projection system in the discussion 
which follows. 

Arrangement V. For the production of targets 
darker than their backgrounds, a pair of projectors 


(Figures 16 and 17) was used. The second projector 
was necessary to equalize the light projected on the 
screen surrounding the target, at all contrasts. The 
target was projected by placing one of several glass 
plates in the focal plane of projector A, each plate 
bearing an opaque circular spot. The projected di¬ 
ameters of these spots subtended angles of 5.01, 
9.55, 18.9, 55.5, and 114 minutes at the eyes of the 



Figure 11. Shutter and buzzer signal switch. 


observers. To procure projection at eight points on 
the screen, the plateholder was supported on pins 
set in two sprocket gears as shown in Figure 18; the 
position of the target was indicated by the same 
electric arrangement as was used for the bright tar¬ 
gets. Projector B illuminated the entire screen uni¬ 
formly. 

A filter disk with six apertures, shown in Figure 
19, was mounted with its axis of rotation midway 
between the two projectors. An empty aperture, four 
filters, and an opaque plate gave transmittances of 
1.003, 0.760, 0.595, 0.362, 0.231, and 0.000. Comple¬ 
mentary transmittances were placed diametrically, 
the opaque plate opposite the empty aperture, filter 
0.760 opposite 0.231, 0.595 opposite 0.362. Conse¬ 
quently, five contrasts ranging from a maximum 
down to 23.1 per cent of the maximum could be 
obtained by successively placing the five apertures 
(other than the opaque plate) next to the condenser 
lenses of the projector. 


CONFIDENTIAL 














































RELATIVE TRANSMISSION 


40 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 



WATER 

CELL 


NEUTRAL 

ABSORBING 


APERTURE HAND- 
PLATE OPERATED 


PROJECTION 

LENS 



LENSES FILTERS PRISM 

Figure 12. Projector Arrangement II. for intermediate background brightnesses. 


PROJECTOR FILTERS 

DESIGNATED BY NOMINAL DENSITY 



WAVELENGTH IN MILLIMICRONS 

Figure 13. Spectral transmittance curves for typical 
range filters used in projectors. Numerals on curves 
indicate the nominal density of each filter. 


In this fashion, the total brightness of the sur¬ 
rounding screen was maintained approximately the 
same for all contrasts, which were indicated elec¬ 
trically by a switch similar to that shown in Figure 
10. For most experiments, part of the illumination 
of the screen was provided by lighting units located 
in the troffers behind the side-wall panels. The tar¬ 
get and compensating projectors were extinguished 
while the location and contrast of the target were 
being changed. 

Arrangement VI. A third “masking” projector 
(Figure 20) was employed to avoid considerable 
changes of screen brightness in the intervals be¬ 
tween presentations. This projector illuminated the 
screen uniformly while the others were extinguished. 
By the use of auxiliary filters, each projector was 
adjusted so that the brightness of the screen was 
nearly constant throughout each experiment. 

Arrangement VII. A “fade-in-and-out” shutter 
(Figure 21) was used to avoid sudden flashes when 
the target projector was extinguished and the screen 
illuminated by the masking projector. An auxiliary 
projector (Figure 22) was employed for illuminating 
the first panels of the observation room in some ex¬ 
periments, to reduce the contrast between the dark 
side walls and the front wall when the illumination 
of the latter was provided mostly or entirely by the 
projectors. 

Projector Arrangements IV and V were also used 
for projecting silhouettes of a German Mavis-class 
destroyer and a German Heinkel 111 bomber. These 


CONFIDENTIAL 
























































































































































































INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


41 


PROJECTION 

LAMP 



WATER NEUTRAL HAND 

CELL ABSORBING OPERATED 



PROJECTION 

LENS 



ROTATING 

PRISM 


Figure 14. Projector Arrangement III, for small targets brighter than the brightest background. 


tests verified the substantial equivalence of visibil¬ 
ity of such silhouettes with circular targets of the 
same area and contrast against the background. 

Arrangement VIII. To project a fixed central spot, 
the target was the image of a round hole in an aper¬ 
ture plate, the hole centered on the axis of the 
projector, as shown in Figure 23. The neutral ab¬ 
sorbing filters were selected for each experiment so 
that, at maximum contrast, the target was barely 
perceptible to all observers. Four lower degrees of 
contrast were obtained from the filter disk shown 
in Figure 19. An opaque plate over one of the aper¬ 
tures of this disk was used to prevent the projection 


of the target in frequent instances, so that the pro¬ 
portion of guessing in responses could be determined. 

Small-Target Presentation. An essentially differ¬ 
ent arrangement of equipment was used for experi¬ 
ments with very small targets subtending 0.6 
minute. Projected targets of this size did not have 
suitably sharp definition, nor could their contrasts 
be measured with sufficient accuracy. For these ex¬ 
periments, eight small holes were drilled in the front 
wall of the observation room. Short lengths of glass 
rods, 3 millimeters in diameter, were pushed through 
these holes until their ends were flush with the 
inside surface of the wall. The ends of the glass rods 


PROJECTION 

LAMP 


\ 


W 


WATER NEUTRAL GROUND APERTURE HAND PROJECTION EXIT 

CELL ABSORBING GLASS PLATE OPERATED LENS PUPIL 




LENSES FILTERS PRISM 


Figure 15. 


Projector Arrangement IV, 


for targets lighter than background, with backgrounds of low brightness. 


CONFIDENTIAL 


































































42 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


PROJECTOR A 


PROJECTION 

LAMP 


WATER 


NEUTRAL 


TARGET 


APERTURE 


MOTOR- 


\ 




PROJECTOR B 


MECHANISM 



Figure 16. Projector Arrangement V, for targets darker than background. 


were polished flat, and at a point approximately 
0.01 inch from the outside end of each rod an opal 
glass plate was placed which was illuminated by two 
systems of lamps, shown in Figure 24. The oblique 
illumination of the opal plate by the pair of Mazda 
Number 34 lamps was adjusted so that, with the 
observation screen at the desired brightness, the 
glass rod was invisible even when viewed from a 
very short distance. 

For bright targets, these matching lamps were lit 
at all times. The projection lamp of each unit fur¬ 
nished the increment of illumination w r hich deter¬ 
mined the contrast of the target. Only one of these 
“increment lamps” was operated at a time, so that 
the brightness of only one of the glass rods differed 
from the brightness of the screen. Negative con¬ 
trasts (dark targets) were produced by operating 


all of the “increment lamps,” and by adjusting the 
total illumination of the opal glass so that the glass 
rods were invisible when all lamps in each unit were 
operated. The contrast of the dark target thus pro¬ 
duced was controlled by the illumination of the 
opal glass due to the “increment lamp.” The illumi¬ 
nation provided by this lamp could be changed by 
varying its distance from the opal glass and by the 
use of absorbing filters between the lamp and the 
opal glass. 

Orientation Spots 

For the 8-position target experiments, a red spot 
1 inch in diameter was always visible at the center 
of the observation screen, serving to orient the ob¬ 
servers during experiments with dark backgrounds. 


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INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


43 


The spot, of adjustable intensity, shone through a 
red filter and a hole in the wall, as already men¬ 
tioned. Red was considered to have the least influ¬ 
ence of all colors on adaptation and was most easily 
distinguishable from the achromatic target at all 
adaptation levels. The brightness of the orientation 
spot was adjusted for each experiment so that it was 
only bright enough to be visible with certainty to all 
observers. 

During the single-position experiments, the diam¬ 
eters of the projected targets were 0.7, 2.0, 3.88, 
11.75, 27.88, and 83 inches. The location of the tar¬ 
get was indicated by four orientation spots. These 
were placed in groups of four, above, below, and at 
either side of the target. Within each group, the 
spots were arranged in a straight line from the cen¬ 
ter of the projection screen at distances of 17, 23, 
29, and 56 inches, respectively. This made possible 


the selection of any four equidistant spots, so that 
in any one experiment the spots were more than 14 
and less than 17 inches from the edge of the target. 
Poorer results were obtained when greater and lesser 
separations were tried. 

These orientation spots consisted of glass rods, 3 
millimeters in diameter, inserted in holes in the 
screen and lighted from behind. Their brightnesses 
were adjusted for each experiment so that they were 
noticeable but not disturbing. 

3,25 Recording Apparatus 

Indicators. Each observer was provided with an 
indicator mounted in the right armrest of her seat. 
The indicator (Figure 25) consisted of a large handle 
which the observer turned until it pointed in the 
direction corresponding to the position in which the 


APERTURE 



Figure 17. Projectors (Arrangements V, VI, and VII) assembled for projection of targets darker than background. 


CONFIDENTIAL 




































44 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 



Figure 18. Mechanism for locating opaque spot in Projector A in eight eccentric positions. Two views, showing 
parallel displacement mechanism, positioning handle and indicating switch. 


target was judged to be. Roundheaded tacks 
driven into the arm of the seat just outside the path 
of the handle guided the observer in tactually locat¬ 
ing the switch point in the dark without visual dis¬ 
traction. The handle rotated the contact bar of a 
16-point selector switch, which contained dead 
points separating the eight possible target positions, 
so there could be no ambiguity concerning the indi¬ 
cation by the observer. The handle rotated freely 
in either direction, and the switch was silent to 
prevent an observer from getting clues from clicks 
of neighboring switches. 

Recording Boxes. Indicator points were connected 
to miniature neon-discharge lamps located in three 
duplicate recording boxes in the control room. There 
were 96 lamps in each box in a rectangular array of 
12 rows of 8 lamps each. Five of the lamps in the 
top row were connected to the five switches of the 


filter disk of the projector; these indicated the con¬ 
trast of the target for each observation. The second 
row of lamps was connected to the switches of the 
target-position mechanism and indicated the actual 
position of the target. Each remaining row of lamps 
was connected to the indicator switch of one ob¬ 
server, showing that observer’s judgment as to the 
location of the target. 

The lamps were separated by sheet-metal parti¬ 
tions, forming a rectangular grid and covered with 
a sheet-metal plate with a ] /i-inch hole located over 
the center of each bulb. Above this metal plate, a 
piece of plate glass, flush with the top of the table 
containing the recording box, carried guides against 
which a sheet of thin paper could be accurately 
positioned. A separate sheet of paper was placed on 
the glass for each presentation of the target. The 
record assistant marked the paper above each glow- 


CDNFIDENTIAL 






























INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


45 


ing lamp (Figure 26). The duplicate recording boxes 
provided facilities for relief of the recording assist¬ 
ant and were spares in case any of the neon lamps 
failed. 

The recording sheets were numbered consecu¬ 
tively. For analysis, they were sorted according to 
contrast, indicated by the position of the mark in 
the first row, and the number of correct judgments 
for each observer was counted for that contrast. 
Incorrect judgments were indicated by marks in 
columns other than the one marked in the second 
row. In experiments with fixed central targets, the 
handle was placed at one position (Figure 25) when 
the target was seen and in another when not. 

3 - 26 Observers 

Selection of Observers 

Very careful consideration was given to the selec¬ 
tion of observers. Although it was necessary to have 
observers with vision as good as that of naval look¬ 
outs, the use of young men of service age was inad¬ 
visable. The number of suitable males available 
\yas limited, and there could be no assurance of their 
continued availability for the duration of the pro*- 
gram. Since there is no evidence of a correlation of 
visual acuity with sex, young women were employed 

PROJECTOR C 


as observers. Also, although observations did not 
occupy all of the working hours and although as¬ 
sistance was needed in many technical tasks con¬ 
nected with the project, it was impracticable to 



PROJECTION 

LAMP 



WATER NEUTRAL 



MOTOR- PROJECTION 
DRIVEN LENS 

SHUTTER I 



IN CONJUNCTION WITH PROJECTORS A AND B 

Figure 20. Projector Arrangement VI, for projecting an equivalent background during the period for changing posi¬ 
tion and contrast in Arrangement V. 


CONFIDENTIAL 














































46 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 




Figure 21. Motor-driven shutter for alternating Projector Arrangements V and VI. 


PROJECTOR D 


PROJECTION 

LAMP 

\ 



WATER 


NEUTRAL 


MOTOR- 


PROJECTION 



IN CONJUNCTION WITH PROJECTORS A, B, AND C 

Figure 22. Projector Arrangement VII, for illuminating first panels of observation room. 


CONFIDENTIAL 








































































INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


47 


PROJECTION WATER NEUTRAL APERTURE MOTOR- 



specify women with technical or scientific training. 
College graduates with promising personalities and 
good academic records were selected. Almost with¬ 
out exception these young ladies proved adaptable 
and useful also as computers and technical assist¬ 
ants. All observers were citizens of the United 
States. 



/•GLASS RODS 


-GROUND GLASS 


Figure 24. Illumination arrangement for glass-rod 
targets. 


Visual Requirements 

Visual acuity better than 20-20, without glasses, 
was required of all observers. Although complete 
ophthalmic examination of the conventional type was 
given each observer, no significant correlation was 
found between the conventional data and the results 
of the visibility tests which are the principal sub¬ 
ject of this chapter. 

Living Arrangements 

Since it was necessary for the observers to reside 
at the Foundation, prior experience with dormitory 



Figure 25. Indicator switch at observer’s seat. 


CONFIDENTIAL 





















































48 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 



Figure 26. Recording table and method of use. 


life was considered in the selection of the observers. 
Although not explicitly recognized when the pro¬ 
gram was planned, it became evident that the iso¬ 
lated, almost cloistered, life of the observers and 
the academic atmosphere maintained in the labora¬ 
tory contributed in large measure to the success of 
the program. The observers promptly settled down 
to the routine with quite stable levels of visual 
attainment, and it was possible to verify results even 
after a lapse of many months. 

Motivation 

The task of observing targets of liminal size and 
contrast every day for months in the same surround¬ 
ings and frequently in nearly total darkness was 
very monotonous and exacting. Without a high de¬ 
gree of motivation, the observers probably could not 
have continued the task with a stable level of attain¬ 
ment. The motivation was to a large extent self- 
created and consisted of a lively interest in the 
results of the daily experiments. The observers as¬ 


sisted and took much of the responsibility for the 
tabulation and analysis of the results. Consequently, 
each observer was continually aware of her attain¬ 
ment relative to the other observers and to the 
average of the group. Although no emphasis was 
placed on competition, each observer took some 
interest in maintaining, if not improving, the stand¬ 
ard of her results relative to the results of the group. 
After the first week or so, each observer reached her 
ultimate level of attainment and subsequently main¬ 
tained that level with remarkable consistency. The 
consistency of the individual observers fully justi¬ 
fies the procedure, which permitted them to have 
knowledge of their own results. 

The observers were impressed by the interest 
shown by official visitors who indicated the need 
for the information expected to result from the 
research. Several naval officers talked to the group 
of observers and other assistants, and this testi¬ 
mony concerning the immediate practical impor¬ 
tance of this research resulted in marked revival in 


CONFIDENTIAL 







INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


49 


the interest and morale of the group. The presence 
in the staff of several wives and fiancees of overseas 
servicemen served to sustain the patriotic motiva¬ 
tion during the long and tedious months required for 
the investigation of the tremendous range of visual 
conditions of importance to the Services. 

During a delay in the program (caused by instal¬ 
lation of a new heating system in the building), five 
members of the group were transferred for a month 
to the laboratories of the Eastman Kodak Company 
in Rochester. These girls served as observers in the 
related investigation of the effect of color contrast 
on visibility (Section 3.3). They returned to the 
Tiffany Foundation with a new understanding of 
their part in war research and helped infuse new 
enthusiasm in the larger group. 

Constancy of Personnel 

The staff of observers suffered some losses and 
required some replacements during the 16 months 
in which the final data were obtained. These changes 
were not numerous, however, and the average of the 
results from the group did not exhibit serious 
changes as a result of the changes in personnel. The 
group of observers was so nearly homogeneous that 
the average of the results for the group was not 
changed appreciably by occasional absences of one 
or two observers. The reader can demonstrate this 
to his own satisfaction by omitting the data for one 
or two observers from any experiment. 

Use of Nontechnical Personnel ‘in Scientific 
Research 

Although the equipment was designed and the 
procedures developed under the direction of a num¬ 
ber of scientists and engineers employed by the Tif¬ 
fany Foundation at various times, the conduct of 
the experiments was the direct responsibility of re¬ 
cent college graduates. Most of the experimental 
procedures and analyses were performed by the ob¬ 
servers and additional women assistants, few of 
whom had any technical training. The use of this 
very low ratio of staff members with technical 
training to those without was an interesting and, on 
the whole, successful experiment. 

Training of Observers 

The observers were required to indicate a definite 
judgment ,about every presentation of the target, 
regardless of difficulty or consciousness of failure. 


Each observer was told, and learned also by experi¬ 
ence, that such judgments were correct much more 
frequently than she suspected. Consequently, the 
observers gradually developed an attentive, but 
quite detached, attitude in which they searched dili¬ 
gently but were not discouraged when the target 
seemed to be hopelessly invisible. They based their 
indications on the slightest suspicion. 

Obviously, some experience was required to con¬ 
vince each observer of the efficacy of this attitude, 
and considerable practice was needed to develop it 
fully. Consequently, the liminal contrast for a new 
observer, initially much greater than those of ex¬ 
perienced observers, decreased rapidly during the 
first week or two of service. Therefore, results fur¬ 
nished by new observers were not used until they 
became stable. The necessity and adequacy of train¬ 
ing, consisting of at least 18 practice sessions with 
a variety of target sizes and background brightness, 
were established by an investigation conducted at 
the beginning of the program. The training proce¬ 
dure consisted of letting the new observer work with 
the group, showing her how her scores compared 
with her impressions and with the scores of her 
companions, and calling her attention to improve¬ 
ment of her scores relative to those of the group as 
she learned and cultivated the detached, confident 
attitude. This attitude was encouraged solely be¬ 
cause it gave the most reproducible results, and to 
this feature of the procedures may be attributed the 
high degree of self-consistency of the final results, 
as indicated by the closeness with which the experi¬ 
mental points fit smooth and regularly spaced 
curves. 

Although the observations required great effort 
and concentration during the first practice sessions, 
experience yielded stable and efficient performance 
with a minimum of strain. Conversation and recep¬ 
tion of radio programs were permitted during the 
observations and appeared to promote rather than 
interfere with consistent observations. Experienced 
psychophysicists expect to get the most consistent 
results from practiced observers who make their 
judgments automatically without full conscious 
awareness of the process. 

3 ' 2 ' 7 Experimental Procedure 

Study of previous investigations and experience 
during preliminary experiments indicated that much 


CONFIDENTIAL 



50 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


of the inconsistency and uncertainty of data on 
visual acuity and visibility could be attributed to 
inaccurate photometry. Inadequate methods of pho¬ 
tometry have been tolerated in many previous 
investigations because of a belief that the observa¬ 
tions were not sufficiently reproducible or self- 
consistent to necessitate very accurate photometry. 
Adoption of the methods of observation described 
above and the use of a large homogeneous group of 
observers secured a degree of observational repro¬ 
ducibility and accuracy never before imagined pos¬ 
sible in visibility experiments. Consequently, con¬ 
ventional methods of photometry, especially for 
the measurement of low contrast and low bright¬ 
ness, were found to result in errors which were seri¬ 
ous in comparison with the precision of the visibility 
observations. Irregularities in the curves represent¬ 
ing the results (of liminal contrast versus adapta¬ 
tion brightness), many times larger than the prob¬ 
able error of the observations, were finally proved 
to be consequences of inaccurate photometry. The 
methods of projection and of photometry were com¬ 
pletely revised before the final set of experiments 
was undertaken, so that the accuracy of photometric 
specification is equal or superior to the accuracy of 
the observational limens. 

Photometric Procedures 

All of the photometric procedures used in this 
research were based on the use of standard lamps 
and the inverse square law of illumination. A Mac¬ 
beth iiluminometer 9a was employed as a compari¬ 
son device, but no reliance was placed on its cali¬ 
bration and its scale was used only to determine the 
ratio of nearly equal brightnesses. A photocell pho¬ 
tometer was used only in the study of the uniformity 
of illumination of small portions of the screen. The 
Macbeth iiluminometer was fitted with a telescopic 
attachment for studies of the uniformity of bright¬ 
ness over larges areas of the screen, walls, floor, and 
ceiling. In all cases, however, absolute values of 
brightness were determined by setting up an equiva¬ 
lent brightness by use of a standard lamp at a meas¬ 
ured distance from a standard reflecting surface. 

In some cases the standard lamp could not pro¬ 
duce a sufficiently high or low brightness, and in 
these cases different filters had to be used in the 
iiluminometer during photometric matching of the 
screen and the test plate. The transmittances of 
these filters were calibrated in the iiluminometer by 


direct application of the inverse square law, using 
the standard lamp and test plate. 

Standard Lamps. Three sets of three standard 
lamps (approximately 1,000, 300, and 20 candle 
power) were used during the program. Four of these 
lamps were calibrated at the beginning of the ex¬ 
periments by the Electrical Testing Laboratories, 
2 East End Avenue, New York, and were used only 
for periodic recalibration of other lamps, which 
were used as working standards in the routine pho¬ 
tometry. The working standards were calibrated by 
moving them to such distances from a test plate 
that they produced the same brightness as the ref¬ 
erence standard lamps at known distances. Simi¬ 
larly, the brightness of the projection screen for 
various experimental arrangements w^as determined 
by producing an equal brightness with a working 
standard lamp at a measured distance from a test 
plate of known reflectance. Each set of standard 
lamps was enclosed in a boxlike carriage (Figures 3, 
27, and 28) which could be moved on a track to any 
desired distance from the standard test plate. The 
interior of the carriage was painted black and was 
provided with several baffles which eliminated stray 
light. The lamps were operated from storage bat¬ 
teries and at the voltage specified by the Electrical 
Testing Laboratories. 

The Test Plate. Since there was no direct way of 
determining the reflectance of the projection screen 
(front wall of the observation room), a standard 
test surface was illuminated by the standard lamps 
(Figure 28). The brightness of this surface was com¬ 
puted on the basis of its distance from the standard 
lamp and its reflectance. The test plate was a piece 
of opal glass, 8 inches in diameter, with an acid- 
etched surface. This was supplied by the Electrical 
Testing Laboratories, which certified the reflectance 
for normally incident illumination. The brightness 
of this surface was constant within 0.2 per cent for 
all angles of observation within 15 degrees of the 
normal, and the plate was never used at greater 
angles. The test plate was mounted in the center of 
a large black screen in order to minimize re-illumi¬ 
nation by light reflected from the white walls of the 
observation room. 

For measurements of brightness of 0.1 foot- 
lamberts and greater, the Macbeth iiluminometer 
was provided with a telescopic attachment (Figure 
29). This consisted of a lens mounted in a tube so 
that an image of an area 4 inches in diameter, at a 


CONFIDENTIAL 




INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


51 


distance of 60 feet, was imaged sharply in the com¬ 
parison field of the illuminometer. Some light from 
other areas of the projection screen also contributed 
to the brightness of the photometric field of the il¬ 
luminometer because of lens flare and internal re¬ 
flections in the telescopic tube. This stray light was 
reduced, but not completely eliminated, by installa¬ 
tion of baffles in the telescope. 



During calibrations, the telescope was used in 
exactly the same manner as for measurement of the 
brightness of the projection screen. Since the test 
plate used with the standard lamp was only 8 inches 
in diameter and was mounted in the center of a 
black screen, no appreciable stray light was present 
during calibration. Consequently, an error was 
caused by the stray light which was present during 
the determination of screen brightness. This error 
'was evaluated by experiment and found to be 1.5 
per cent of the brightness of the surrounding screen. 
There was no appreciable stray light during meas¬ 
urement of the projected light targets because the 
surrounding screen was dark during these measure¬ 
ments. Consequently, no correction was needed in 
the determination of the brightness increments of 
bright targets. 

Low-Level Photometry . Special methods were de¬ 
vised for the accurate determination of screen bright- 



Figure 28. Photograph of arrangement of standard 
lamps, test plate, and black background for photom¬ 
etry of observation room. 


CUBE 



nesses below 0.1 foot-lambert. These consisted 
of measuring the brightness of the ground glass 
of the troffer lighting units, as shown in Figure 30, 
and the brightness of the opal glass plate in Pro¬ 
jector Arrangement IV (Figure 15), as seen through 
the lens. Factors were determined by which the 
brightness of each source could be multiplied in 
order to compute the brightness produced on the 
screen. These factors were determined at inter¬ 
mediate levels of screen brightness for which direct 


CONFIDENTIAL 


















































52 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


photometry was satisfactory, and the arrangements 
of the sources were kept strictly unchanged for 
lower levels for which the indirect photometry was 
applied. 

A psychometric determination of a low-level 
brightness match provided an independent verifi¬ 
cation of the foregoing method. For this test, a gray 
disk (diameter 36 inches) was mounted in the center 
of the white front wall of the observation room to 
form a photometric field. When the room was illu¬ 
minated with the troffer lamps, the gray disk ap- 



Figure 30. Macbeth illuminometer arranged for pho¬ 
tometry of low-level troffer lighting-units. 


peared darker than the surrounding white screen. 
Additional light was put on the disk alone by use 
of Projector Arrangement IV. Two slightly differ¬ 
ent degrees of illumination were provided by the 
projector. In one case, the disk was just brighter 
and in the other it was just darker than the sur¬ 
rounding white screen. 

These increments of illumination were projected 
on the gray disk 64 times each in a random order. 
The observers were asked to indicate for each pres¬ 
entation whether they judged the disk to be brighter 
or darker than the surrounding screen. The propor¬ 
tions of judgments “brighter” were plotted against 
a quantity proportional to the brightness of the 
disk. This quantity was the brightness of the sec¬ 
ondary source of the projection system, which was 


sufficiently brighter than the disk itself so that direct 
photometry was feasible for all adaptation levels 
at which the disk was used. The factor of propor¬ 
tionality between the brightness of the secondary 
source and the brightness of the disk was deter¬ 
mined at an intermediate level for which direct 
photometry was adequate. The brightness of the 
screen during the psychometric comparisons with 
the disk was considered to be the brightness of the 
disk interpolated so as to correspond to 50 per cent 
judgment of “brighter.” The brightness of the screen 
determined in this way confirmed the value com¬ 
puted by applying the experimental factors to the 
brightness of the lamp units in the troffers. 

Contrast Determination. In order to determine 
contrasts with the greatest accuracy, only the maxi¬ 
mum increment of brightness was measured directly. 
The four smaller contrasts used in each experiment 
were computed by multiplying the maximum con¬ 
trast by the transmittances of the filters through 
which the incremental light was projected. The 
transmittances of these filters were measured in the 
projector system at levels of brightness most favor¬ 
able for accurate photometry. Consequently, the 
products of these transmittances by the maximum 
contrast in any experiment were more accurate than 
direct measurements of the reduced contrasts. 

Similarly, the brightness increments of the tar¬ 
gets projected in experiments at low levels of adap¬ 
tation were determined by direct photometry with 
all filters removed. The measured value was then 
multiplied by the transmittance of the filter com¬ 
bination used to determine the limen. 

Measurement of Filters. The transmittance of 
each filter combination was determined by measure¬ 
ments at high-brightness levels. For these measure¬ 
ments and for the measurement of the contrast- 
control filters, ground-glass plates were inserted in 
the projector between the filters and the projection 
lens. Various numbers of ground-glass plates were 
used in different instances to provide a brightness 
level favorable for accurate photometry. The bright¬ 
ness of the foremost of these plates, as seen through 
the lens, was measured by use of the telephotometer 
and the standard-lamp method of evaluating each 
brightness setting. The brightness of the foremost 
ground-glass plate was measured with and without 
the filter in the projector. The ratio of the bright¬ 
nesses was taken as the effective transmittance of 
the filter. 


CONFIDENTIAL 























INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


53 


Uniformity of the Screen. Very slight variations 
of the observed contrast, depending on the target 
position and location of the observer, were caused 
by the slightly glossy surface of the screen. These 
variations were measured with the telephotometer. 
A method was evolved which permitted this second- 
order effect to be allowed for in interpreting the 
data. 

The effect of various target sizes on the projected 
brightness was also studied, and there was found 
to be no significant difference. The distribution of 
brightness over the area of the larger targets was 
studied and found to be quite uniform. The effective 
area of the smallest targets was determined by 
measurement of the total flux in the image. The 
angles subtended by the smallest targets were com¬ 
puted from these results to compensate for imper¬ 
fect image projection and for stray light at the 
edges of the image. 

Photometry of Small Targets. A lens was used, 
as shown in Figure 31A, to image the ends of the 
glass rods in the field of the Macbeth illuminometer, 
for routine measurements of their brightnesses. The 
effects of minute imperfections on the ends of the 
glass rods, and of the holes through which they were 


inserted, were evaluated by use of the flux photom¬ 
eter shown in Figure 31B. The end of each glass rod 
and its immediate surroundings were imaged in the 
aperture of the cavity. The interior of this cavity 
was whitened, and the brightness of the portion of 
the interior viewed through the photometer was 
proportional to the flux entering the cavity. The 
average brightness of each glass rod was taken to 
be the brightness of a large uniform surface which 
produced the same reading in the flux photometer, 
multiplied by the ratio of the area of the opening 
of the photometer cavity to the area of the image 
of the end of the glass rod focused in the aperture 
of the cavity. 

Computation of Contrast. In all cases, contrast 
was computed by dividing the difference of bright¬ 
ness between the target and background by the 
brightness of the background. Targets brighter than 
their backgrounds, therefore, had contrasts ranging 
from zero to infinity, while targets darker than their 
backgrounds had contrasts ranging from zero to 
one. Since the difference of brightness was produced 
by projection, this increment was measured directly. 
This avoided the gross errors which result from sub¬ 
tracting two nearly equal quantities, both of which 


A 



Figure 31. (A) Macbeth illuminometer arranged for routine photometry of glass-rod targets. (B) Macbeth illumi¬ 

nometer arranged for evaluating average brightness of glass-rod targets. 


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54 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


would be subject to errors comparable in magnitude 
to their difference. 

3,2 8 Psychometric Procedure 

Criterion of Visibility 

The basic experimental task was to determine the 
contrast which produced a standard degree of visi¬ 
bility for each size of target and brightness of back¬ 
ground. The standard degree of visibility, or cri¬ 
terion for identification of the contrast just visible, 
was that for which the location or presence of the 
target was reported correctly just 50 per cent of the 
time, due allowance being made for accidentally 
correct or chance reports. For example, when there 
were eight possible positions of the target, coinci¬ 
dence of random guesses would be expected to yield 
one correct report out of every eight responses. The 
criterion of visibility adopted in this investigation 
required half of the remaining 87% per cent of re¬ 
ports to be correct. These, in addition to the 12% 
per cent attributed to chance, correspond to 56% 
per cent correct responses, which was the score em¬ 
ployed for the determination of liminal contrasts 
throughout this investigation. 

This degree of success corresponds to a very low 
degree of self-assurance but gives the most accurate 
and statistically reliable determination of the de¬ 
pendence of visibility on contrast and adaptation 
brightness. Liminal contrasts, or target sizes, deter¬ 
mined by use of this criterion in the unstrained con¬ 
ditions of the laboratory, may or may not be less 
than the contrasts and sizes of targets sighted under 
the conditions of discomfort and violent disturbance 
but extremely great motivation of naval lookouts. 
It can be safely assumed, however, that any change 
of contrast or background brightness will produce 
a change of range of detection accurately propor¬ 
tional to the change of range predicted on the basis 
of the laboratory data. The use of the criterion of 
visibility and conditions of observation which yield 
the most accurately consistent laboratory data is, 
therefore, justified and necessary. 

Target Presentation 

Each experiment used a single size of target and 
a single brightness of background. The target was 
exhibited with five different contrasts, the ratios of 
which are specified in Section 3.2.3. It suffices here 
to mention that these fixed ratios, with the mini¬ 


mum contrast approximately one-quarter of the 
maximum, were found suitable for the determina¬ 
tion of adequate psychometric curves for all target 
sizes and at all levels of adaptation. For each size 
of target and background brightness, a short pre¬ 
liminary series of observations was made to confirm 
or correct the choice of the maximum contrast. The 
requirement on this adjustment was that the most 
sensitive observers should correctly report the loca¬ 
tion of the target for less than 56 per cent of the 
presentations at lowest contrast, and that the least 
sensitive observers should correctly report the loca¬ 
tion of the target for more than 56 per cent of the 
presentations at highest contrast. For groups of 
nearly equally sensitive observers, the target was 
reported correctly for 92 to 97 per cent of the pres¬ 
entations at maximum contrast and for 13 to 22 
per cent at lowest contrast. 

Order of Presentation. The five contrasts were 
presented in random order. The order for each ex¬ 
periment was governed by a schedule prepared in 
advance by recording the order in which numbered 
cards occurred in a shuffled deck. This deck con¬ 
sisted of 16 sets of 5 cards, each numbered from 
1 to 5. Preparation of the schedule from this deck 
assured that each contrast would be presented 16 
times during each group of 80 presentations. This 
deck was reshuffled after the schedule for each 80 
observations had been copied. 

For the 8-position experiments, the position of 
the target was also governed by a prearranged 
schedule determined by means of five shuffled decks 
of 64 cards each. One of the five decks was assigned 
to each contrast; and within each deck the cards 
were marked “North,” “Northeast,” “East,” et 
cetera. Whenever a given contrast was called for 
by the deck of cards described in the preceding 
paragraph, the top card was taken from one of the 
five position-indicating decks. Thus, targets having 
each of the five contrasts were presented in each 
of the eight positions eight times. This series of 
presentations constituted one experiment. In order 
to provide rest periods, each series of 320 observa¬ 
tions was divided into four groups of 80 observa¬ 
tions each. 

Rest Periods. Rest periods were scheduled 
throughout each experiment. In the case of the 
8-position tests, the regular schedule included rests 
of 5 minutes’ duration, with the observers remaining 
in the observation room, between the first and sec- 


CONFIDENTIAL 





INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


55 


ond and between the third and fourth quarters of 
the experiment. A rest of at least 10 minutes outside 
the room was allowed at the end of the second quar¬ 
ter, followed by 10 minutes’ adaptation in the room 
before the third group of observations was com¬ 
menced. The observers were adapted for 5 minutes 
before beginning each group of observations at 10 
and 100 foot-lamberts. For experiments in which 
the brightness of the background was 10“ 2 foot- 
lambert or lower, the observers wore standard Navy 
dark-adaptation red goggles during their rest period 
outside the observation room. The schedule was 
flexible within limits which assured adequate rest 
and adaptation. The normal duration of a group of 
80 observations (16 minutes) was sometimes length¬ 
ened by delays for adjustment of voltage or other 
contingencies. Since these delays gave the observers 
some unscheduled rest, their scheduled rest periods 
in such cases might be reduced slightly. The total 
duration of the first or third quarters, plus the en¬ 
suing rest in the observation room, was never less 
than 21 minutes, however, and the rest period after 
these quarters was never less than 2 minutes. Occa¬ 
sionally the available time was used to better ad¬ 
vantage by running the first half of an experiment 
before a meal and the second half afterwards. Some¬ 
times, by common consent, the observers remained 
in the observation room between halves, so as to 
save the time required for re-adaptation. The time 
at which each portion of the experiment was begun 
and finished was recorded. 

Sequence of Operations. Each presentation of the 
target involved a series of operations. During the 
interval between presentations, the filter wheel was 
rotated so as to place in the projector the filter 
specified by the schedule. The operator watched the 
second hand of an electric clock and at the appro¬ 
priate instant opened the shutter. This operation 
caused a buzzer to sound as long as the shutter re¬ 
mained open, informing the observers that the target 
was on the screen. At the end of the exposure, the 
operator closed the shutter and adjusted the filter 
wheel and target position for the next presenta¬ 
tion. 

Recording of Data. During each presentation of 
the target the observers indicated their judgments 
of the location of the target by turning their 
switches to the corresponding positions. The record¬ 
ing assistant placed a tissue sheet, numbered ac¬ 
cording to the serial number of the presentation, on 


the recording box and marked around each lighted 
lamp with a pencil, thus recording the actual con¬ 
trast and position of the target and the judgments 
of the observers. 

Analysis of the Data 

Eight-Position Experiments. When the experi¬ 
ment was complete, the 320 record sheets were 
sorted according to contrast. The number of correct 
judgments was counted for each observer and for 
each contrast. The proportion of correct responses 
was computed by dividing the number of correct 
responses by the total number of presentations. 
These proportions were computed for each observer 
and for the group as a whole. From the proportions 
of correct judgments at the 5 degrees of contrast, it 
was possible to plot psychometric functions for the 
several observers and for the group of observers 
as a whole. The limen for any observer was the 
value of contrast corresponding to 36 correct re¬ 
sponses in a series of 64 presentations, a proportion 
of 0.5625. The limens were computed by the Urban 
constant process, 28 rather than by graphical or 
linear interpolation, in order to employ all of the 
observational data rather than only those for the 
two contrasts yielding most nearly liminal re¬ 
sponses. 

Single-Position Experiments. In the single-posi¬ 
tion experiments, the liminal contrast was deter¬ 
mined by interpolation between the scores for the 
contrasts actually used. It was that contrast for 
which each observer reported the presence of the 
target correctly in 50 per cent of the instances, due 
allowance being made for guessing. Guessing that a 
target was present, even though not visible, was dis¬ 
couraged by the knowledge that in frequent in¬ 
stances no target was projected. The residual effects 
of guesswdrk were at least partially compensated by 
dividing the incorrect scores for all contrasts by the 
fraction of target absences which were correctly 
reported as such. The proportion of correct re¬ 
sponses was consequently reduced by this compen¬ 
sation. The consistency of the results was improved 
because the amount of guessing by each observer 
fluctuated much more violently from day to day 
than the adjusted scores for the several contrasts. 

The single-position experiments usually involved 
about 200 observations. Greater numbers of obser¬ 
vations would be expected to improve the precision 
of the results, but in most cases they could not be 


CONFIDENTIAL 



56 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


obtained before the observers became fatigued. 
Each of the single-position experiments included 
equal numbers of presentations for two quite differ¬ 
ent observation times. These two sets of presenta¬ 
tions were intermingled in random order, so that the 
effects of fatigue would be equalized. The results 
for the two sets were compared, and their approx¬ 
imate equality was taken as proof that both obser¬ 
vation times were sufficient. The results for the two 
sets were averaged to obtain the final values. 

Record of Research 

The determination of liminal contrast for each 
target size and field brightness was considered a 
separate experiment to wdiich at least one full ob¬ 
servation period was devoted. Separate reports of 
procedure, photometric data, and observational re¬ 
sults were prepared for each experiment, and the 
complete record is a document of more than 2,700 
pages, describing over 200 separate experiments. A 
complete copy of the record is included in the micro¬ 
film supplement to this volume. 23 The report of 
each experiment contains a tabular summary which 
includes the proportion of the responses correct for 
each observer and each contrast; the liminal con¬ 
trasts for each observer computed by the Urban 
method; the liminal contrasts for the group, com¬ 
puted as the arithmetic mean of the liminal con¬ 
trasts for the individual observers; and the standard 
deviation of the liminal contrast for the group. 
Table 1 is an example of such a summary. 


3 29 Summary of Results 

Eight-Position Experiments 

The liminal contrasts of round targets brighter 
than their backgrounds, when the targets could ap¬ 
pear in any one of eight positions, are shown in 
Figure 32. The observation time was 6 seconds in 



Figure 32. Liminal contrasts for round targets 
brighter than their backgrounds. 

Eight-positions, 6 seconds observation time. Angular sub¬ 
tense of diameter is shown (in minutes) at right of each curve. 

all cases represented on this diagram. Each curve 
represents the data for a single target size, the 
angular subtense of which is indicated to the right 
of the curve, in minutes of visual angle. The bright¬ 
ness of the background, expressed in foot-lamberts, 
is represented on a logarithmic scale (base 10) along 


Table 1. Psychometric data for a typical 8-position experiment. 

Target: circular; brighter than background; subtense, 121 minutes; back¬ 
ground brightness, IU 4 foot-lamberts. Arithmetic mean = 0.00747; 
standard deviation rr 0 . 00121 . 


Seat 

No. 

Observer 

0.00291 

0.00461 

Contrasts 

0.00670 

0.00945 

0.0124 

Contrast 

limen 

1 

ccc 

0.167 

0.281 

0.444 

0.578 

0.921 

0.00836 

2 

MJB 

0.197 

0.328 

0.587 

0.781 

0.984 

0.00688 

3 

ESK 

0.136 

0.281 

0.460 

0.937 

1.00 

0.00688 

4 

MC 

0.167 

0.281 

0.476 

0.641 

0.952 

0.00795 

6 

ITB 

0.258 

0.578 

0.873 

0.984 

1.00 

0.00469 

7 

MRR 

0.167 

0.234 

0.349 

0.750 

0.905 

0.00826 

8 

VRM 

0.197 

0.453 

0.460 

0.797 

0.952 

0.00693 

9 

ELC 

0.167 

0.141 

0.270 

0.672 

0.889 

0.00896 

10 

SR 

0.121 

0.203 

0.238 

0.781 

0.968 

0.00829 


CONFIDENTIAL 































INFLUENCE OF BRIGHTNESS CONTRAST ON VISIBILITY 


57 


the horizontal axis. The contrasts required for 50 
per cent (above chance) frequency of correct re¬ 
ports by the group of observers is shown on a verti¬ 
cal logarithmic scale. 

A greater number of experiments is indicated in 
the neighborhood of 0.001 foot-lambert than at any 
other adaptation level. These were obtained in order 
to verify the existence of the discontinuity of slope 
indicated by the curves. This discontinuity is at¬ 
tributed to the change from rod-vision, which is 
most effective at low levels of adaptation, to cone- 
vision, which is effective at high levels. 

Repeated Experiments. Seven open circles shown 
in Figure 32 indicate the results of experiments 
which were repeated during the series of dark-target 
experiments. In these check experiments, the bright 
targets were produced by Projector Arrangement V, 
with which at least part of the background bright¬ 
ness is produced by the projectors. These check 
experiments served, therefore, to prove the equiva¬ 
lence of the observing conditions used for bright and 
dark targets. They also indicate the consistency of 
the observers over a span of many months. There 
is one check point for each of the four upper curves, 
and three points on the lowest curve. The point on 
the curve for 18.2 minutes is at exactly 0.001 foot- 
lambert and is so nearly coincident with one of the 
original points that it may not be visible in the 
reproduction of Figure 32. 

Dark Targets. The liminal contrasts of round tar¬ 
gets darker than their backgrounds are indicated 
by the experiment points shown in Figure 33. These 
experiments were exactly comparable with those 
for the bright targets. The curves shown in Figure 
33 were interpolated from Figure 32 for bright tar¬ 
gets having the same sizes as the targets used in 
the dark-target experiments. The closeness with 
which the curves fit the experimental points is an 
indication of the equivalence of light and dark con¬ 
trasts in visibility phenomena. 

Single-Position Experiments 

Figure 34 shows the results for light targets which 
were presented in only one location on the screen 
and which were observed for as long as was required 
under each set of conditions in order to obtain the 
highest possible frequency of correct responses. As 
mentioned in preceding sections, the precision of the 
results was lower than in the eight-position experi¬ 
ments. In order to avoid confusion, the experimental 
points are shown for only the largest and smallest 


targets. Data for all of the targets for all adaptation 
levels are given in Table 2. 


Table 2. Minimal contrasts for visibility; circular 
targets brighter than their background. 


Visual angle 
(min of arc) 

Adaptation 

brightness 

(foot-lamberts) 

Liminal 
contrast 
(arith. mean) 

360. 

71.90 

0.003024 

121. 

70.81 

0.002533 

18.2 

71.36 

0.003448 

9.68 

74.78 

0.008658 

0.595 

71.42 

0.4755 

360. 

10.58 

0.003511 

121. 

11.08 

0.003089 

3.60 

11.08 

0.02759 

0.595 

10.82 

0.8894 

360. 

0.8898 

0.002490 

121. 

0.8868 

0.003620 

55.2 

0.8859 

0.003664 

18.2 

1.174 

0.005081 

3.60 

1.095 

0.04745 

0.595 

1.095 

1.835 

360. 

0.09108 

0.005061 

121. 

0.09879 

0.006182 

55.2 

0.1014 

0.006707 

18.2 

0.1038 

0.01070 

9.68 

0.09492 

0.02129 

0.595 

0.09569 

4.658 

360. 

0.01308 

0.009265 

121. 

0.01323 

0.01589 

55.2 

0.01384 

0.02050 

18.2 

0.01346 

0.04764 

9.68 

0.01259 

0.08422 

3.60 

0.01296 

0.4135 

0.595 

0.01278 

13.67 

360. 

0.001128 

0.02770 

121. 

0.001152 

0.04355 

55.2 

0.001192 

0.07093 

18.2 

0.001194 

0.2212 

9.68 

0.001114 

0.6961 

3.60 

0.001084 

3.918 

0.595 

0.001145 

127.2 

360. 

0.0001006 

0.06013 

121. 

0.0001073 

0.09363 

9.68 

0.0001170 

2.663 

0.595 

0.0001104 

570.8 

360. 

0.000008862 

0.1423 

121. 

0.00001063 

0.2696 

18.2 

0.00001043 

3.833 

0.595 

0.00001002 

2.493. 


Although the magnitudes of the liminal contrasts 
were determined more accurately by the single-po¬ 
sition, long-observation technique, the shapes of the 
curves were determined with greater precision with 
the eight-position method. Consequently, curves 
having shapes as similar as possible to those in 
Figure 32 have been fitted to the single-position re¬ 
sults and are shown in Figure 34. Also, it was 


CONFIDENTIAL 







58 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


considered unnecessary to perform single-position 
experiments with targets darker than their back¬ 
grounds, because the precision of the results would 
have been insufficient to detect any differences 



LOG ADAPTATION BRIGHTNESS (FOOT-LAMBERTS) 

Figure 33. Liminal contrasts for round targets darker 
than their backgrounds are shown by indicated points. 

Curves show liminal contrasts for bright targets of same size, 
interpolated from Figure 32. 


which were not detected by the more precise 8-posi¬ 
tion method. 

The dependence on contrast of the visual angle 
subtended by a barely visible circular object is 
shown in Figure 35. The several curves are for dif¬ 
ferent levels of adaptation, which are specified in 
foot-lamberts at the lower ends of the curves. These 
curves were obtained from Figure 34 by graphical 
interpolation and therefore indicate the minimum 
target subtense visible when the location of the tar¬ 
get is known exactly and when the time of search is 
essentially unlimited. The curves in Figure 35 are 
given in tabular form in Appendix A; values from 
this table should be used for all calculations. 


32 10 The Effect of Target Shape 

The effect of the shape of the target on the lim¬ 
inal contrast was studied with the single-position 
method. By these experiments, the liminal contrasts 



Figure 34. Liminal contrasts for round targets brighter than their backgrounds. 

Single position, with sufficient time to attain maximum frequency of correct reports. Experimental points are shown for largest and 
smallest targets. Data for all targets are presented in Table 2. 


CONFIDENTIAL 





































































INFLUENCE OF COLOR CONTRAST ON VISIBILITY 


59 



LOG LIMINAL CONTRAST 


Figure 35. Angular subtense of just visible circles as function of contrast, for various background brightnesses. 

(See Appendix A for a tabular summary of this figure.) 

of squares and circles having the same area were 
found to be equal. Rectangles of various shapes and 
sizes were then studied. The ratio of the liminal con¬ 
trast of a rectangle to the liminal contrast of a 
square or circle having the same area (for the same 
background brightness) is called the form factor. 

The form factor is always unity or greater. Experi¬ 
mentally determined form factors are shown by 
points in Figure 36 for a background brightness of 
10 foot-lamberts and in Figure 37 for an eye adapted 
to 10" 5 foot-lambert. 

Form Factor Theory 

The curves in Figures 36 and 37 represent the 
variation of form factor with angular area predicted 
on theoretical grounds that may be summarized 23a 
thus: 

The liminal contrast of a rectangle is the geomet¬ 
ric mean of the liminal contrasts of squares having 
sides equal in visual target subtense to the sides of 
the rectangle. 

It is to be noted that the side of a square of equal 
area subtends 0.886 times the angle subtended by 
the diameter of a circle. Since the liminal contrasts 
of a square and circle of equal area are equal, the 
liminal contrast of a square is indicated in Figure 
35 by the abscissa of the curve for the appropriate 


adaptation level for the ordinate obtained by 
adding 0.052 to the logarithm of the angle sub¬ 
tended by the side of the square. Consequently, the 
predicted liminal contrast of a rectangle is indicated 
on the logarithmic abscissa scale of Figure 35 by the 
midpoint between the abscissas corresponding to 
the angular subtenses of the sides, that is, at ordi¬ 
nates equal to 0.052 plus the logarithms of the visual 
angle (minutes) subtended by the sides. 

33 INFLUENCE OF COLOR CONTRAST 
ON VISIBILITY 

In the hope of discovering a direct correlation 
between the effects on visibility produced by color 
contrast and brightness contrast, the Eastman 
Kodak Company was asked to study the visual 
acuity of a homogeneous group of observers to 
whom both colored targets and gray targets were 
presented. In these experiments, the acuity obtained 
with any color contrast was specified by the bright¬ 
ness contrast which produced the same acuity. The 
utility of this mode of specification arises from the 
fact that it is nearly independent of differences 
among normal observers, shape of target, and adap¬ 
tation level (at least between 10' 2 and 100 foot- 
lamberts) . 


CONFIDENTIAL 


























60 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 



0 I 2 

LOG TARGET AREA (SQUARE MINUTES) 


Figure 36. Form factors for rectangles. 

Circles, 100:1, triangles, 10:1, squares, 4:1. Brightness of background: 10 foot-lamberts. Curves represent theoretical variation of 
form factor with size. 


By specifying the influence of chromatic contrast 
on acuity and visibility in terms of the equivalent 
achromatic contrast, the results reported earlier in 
this chapter can be extended to include the general 
case of combined chromatic and brightness contrast, 
presumably for all levels of adaptation. 11 

3-31 Earlier Investigations 

Earlier studies of the influence of color on the 
perception of form have been fragmentary and al¬ 
most entirely qualitative. The essential phenomena 
have all been described previously but without 
quantitative specification. Langley (1889) 29 pro¬ 
posed a method of heterochromatic photometry 
based on the fact (or assumption) that visual acuity 
is a function of brightness only, regardless of the 
color of the light. The validity of this method was 
tested and confirmed by Bender (1919), 30 who de¬ 
termined spectral luminosity curves for several ob¬ 
servers and compared these with the curves obtained 

b This investigation was not intended to explore the effects 
of visual angle, adaptation, and contrast on the recognition 
of colors as such. 


for the same observers by flicker photometry. In 
these experiments a Snellen-type vision test chart 
was illuminated with monochromatic light, the in¬ 
tensity of which was varied until a standard value 
of acuity was obtained. Brightness contrast alone, 
albeit with highly chromatic light, was involved in 
these experiments. 

Acuity with patterns involving chromatic differ¬ 
ences was indirectly studied by Lehmann (1904), 31 
Benussi and Liel (1904), 32 Liebmann (1927), 33 and 
Koffka and Harrower (1931). 34 None of these 
studies w r as quantitative, but the evidence was con¬ 
clusive that chromaticity difference alone produces 
very little visibility in the absence of brightness dif¬ 
ferences. Liebmann reported that forms are difficult 
to perceive when the brightnesses of the figure and 
ground are equated, although color differences them¬ 
selves are seen most clearly under these circum¬ 
stances. Koffka and Harrower studied this appar¬ 
ent anomaly, confirmed the difficulty of perceiving 
forms consisting of colors different from but equated 
in brightness to their backgrounds, and reported a 
difference in this respect between the colors akin 
to blue and colors akin to red. Thus the Liebmanp 



LOG TARGET AREA (SQUARE MINUTES) 

Figure 37. Form factor for rectangle 400 minutes long, 4 minutes wide, on background brightness of 10 -5 foot-lam- 
bert. Curve shows theoretical variation. 


CONFIDENTIAL 






















































































INFLUENCE OF COLOR CONTRAST ON VISIBILITY 


61 


effect was most complete, that is, the figure was 
least visible for blue, green, and violet figures on 
equally bright, nearly neutral grounds. Conversely, 
red, red-purple, and orange figures were seen most 
easily on equiluminous backgrounds. Present indi¬ 
cations are that this difference is attributable to 
the much higher purities of obtainable red, red-pur¬ 
ple, and orange colorants compared to the purities 
of moderately bright blue, green, and violet mate¬ 
rials. The disturbing influence of this circumstance 
can best be detected and eliminated by quantitative 
specification of the colors employed. This is one of 
the objectives of the present investigation. Lieb- 
mann, and Koffka and Harrower relied on verbal de¬ 
scriptions of the subjects’ perceptions, which, al¬ 
though interesting and instructive, are not useful as 
a basis for estimating the effect of chromatic con¬ 
trast on visual acuity. 

Acuity was studied with Landolt ring test pat¬ 
terns made from Ostwald colored papers by Hart- 
inger and Schubert (1940) 35 and Schaefer, Kliefoth, 
and von Wolff (1943). 36 The avowed purpose of 
these investigations was to determine the influence 
of colored spectacles on visibility and acuity for 
patterns containing chromatic contrasts. No effort 
was made to eliminate brightness contrasts, and it 
is not clear from the accounts whether the bright¬ 
ness difference was eliminated in a single instance. 
The colors were specified only in terms of the Ost¬ 
wald notations of the papers used, and specifications 
of the influence of the spectacle glasses on the col¬ 
ors were confined to their effects in changing bright¬ 
ness contrasts. Little can be gleaned from these ex¬ 
periments as reported beyond the fact that bright¬ 
ness contrasts exert a predominating influence on 
acuity even when combined with considerable chro¬ 
matic contrast. 

A report by Langstroth, et al. (1943), 37 Visibility 
of Targets in Relation to Night Screening (where 
screening denotes concealment) presents the results 
of tests with large targets (53 minutes minimum vis¬ 
ual angle), low levels of brightness (none greater 
than 1 foot-lambert), in which simple judgments of 
disappearance were reported. The results for gray 
on gray indicate a contrast limen of less than 1 per 
cent for large targets when the brightness is 1 foot- 
lambert or greater. The contrast limen is reported 
to increase from 1 to 10 per cent when the bright¬ 
ness is decreased from 10 _1 to 10" 3 foot-lambert. 
The limen for dark on light is reported to be the 


same as for light on dark. Contrast limen is said 
to be nearly independent of size for all targets sub¬ 
tending 1 degree or more but increases very sharply 
with decrease in size when the target subtends less 
than one-half degree. This effect is much less for 
brightnesses greater than 0.1 foot-lambert than for 
lower brightnesses. Results of observations on the 
visibility of colored targets at night are all ex¬ 
plained on the basis of the Purkinje shift of the 
luminosity curve. The brightness contrast is com¬ 
puted on the basis of physical rather than psycho¬ 
physical data. These facts indicate that the possi¬ 
bility of contributions of chromatic contrast to 
visibility was not considered, and the report yields 
no information on the relative importance of chro¬ 
matic contrast as compared to brightness contrast 
in determining visibility or acuity. 

3 - 3 ' 2 New Results 

The following diagrams (Figures 39 to 41) show 
for each specified condition of observation the 
achromatic brightness contrast which would be nec¬ 
essary in order to yield the same visual acuity or 
visibility as any selected color contrast. These dia¬ 
grams consist of “contours” in the standard chroma- 
ticity diagrams. Along each contour the equally 
effective achromatic contrast is constant. Such con¬ 
tours are shown for equivalent achromatic con¬ 
trasts (e.a.c.) of 5, 10, 15, 20, and 25 per cent. In 
some cases these contours are incomplete, and in 
others fragments of contours for higher e.a.c. are 
shown. These diagrams are the results of conscien¬ 
tious efforts to establish and represent the facts 
correctly, but in the last analysis the diagrams are 
based largely on personal judgment concerning the 
significance of the experimental data. 

The actual data 22 are very erratic and conflict in 
many details with these diagrams. Reasonable rep¬ 
resentations of the results cannot be obtained by 
any strictly objective or statistical treatment of the 
experimental data. Fluctuating motivation and oc¬ 
casional indispositions of the observers contributed 
to the irregularity of the data and have been taken 
into consideration in manners and extents which no 
statistical treatment of the data would permit. 

Gamut of the Data 

For chromaticities beyond the domain enclosed 
by the contours, estimates may be made by extra- 


CONFIDENTIAL 



62 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 



Figure 38. Standard I.C.I. chromaticity diagram. 

Inner curve indicates the gamut of colors obtainable by a 
typical process of color printing. Most colors of nature lie within 
this gamut. 



X 

Figure 39. I.C.I. chromaticity diagram. 

Contours indicate values of equivalent achromatic contrast of 
equiluminous colors on neutral background. The neutral point is 
indicated by a cross. 

polation, but considerable uncertainty must be at¬ 
tributed to such values. Extrapolation is especially 
unreliable when acuity is in question, because the 


effects of chromatic aberration of the eye become 
serious for chromaticities beyond the range covered 
by the contours. Such extreme chromaticities, how¬ 
ever, are rarely encountered in field conditions. An 
idea of the gamut of colors covered by the e.a.c. 
contours can be gained by comparing them with 
Figure 38, which shows the extreme range of colors 
obtainable by a modern process of color printing. 
More extreme chromaticities are seldom found in 
nature. 

Comparison of Experimental Procedures 

Figures 39 through 41 are all based on tests of 
visual acuity, requiring reports of the location of 



X 


Figure 40. I.C.I. chromaticity diagram. 

Contours indicate values of equivalent achromatic contrast 
of equiluminous colors on “sky blue” background (Munsell: 

PB %). Cross indicates the color of the background. 

the gap in a Landolt broken-circle test pattern. Ex¬ 
periments designed to compare the results obtained 
with Landolt rings and with circular spots (which 
might appear in any one of eight eccentric posi¬ 
tions) were performed very early in the investiga¬ 
tion. Figure 43 shows as functions of excitation 
purity, for various dominant (and complementary) 
wavelengths, the e.a.c. of colors approximately 
equal in brightness to their immediate achromatic 
surround, based on the detection of the presence of 
a small spot. Figure 44 shows similar results ob¬ 
tained under the same conditions with Landolt 


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INFLUENCE OF COLOR CONTRAST ON VISIBILITY 


63 



Figure 41. I.C.I. chromaticity diagram. 

Contours indicate values of equivalent achromatic contrast 
of equiluminous colors on “foliage green’’ background (Munsell: 
GY 5/4). Cross indicates the oolor of the background. 



Figure 42. I.C.I. chromaticity diagram. 

Contours indicate values of equivalent achromatic contrast of 
equiluminous colors on green background (Munsell: G %). Cross 
indicates the color of the background. 



2 0 10 20 30 40 50 60 70 80 

EXCITATION PURITY (PER CENT) 


Figure 43. Equivalent achromatic contrast of colore approximately equal in brightness to their achromatic sur¬ 
round, based on the detection of the presence of a small spot. 

Illuminant: tungsten lamp; color temperature: 3,000 degrees K. 


rings. Figure 44 was derived from Figure 43 by 
plotting the purities corresponding to the indicated 
values of e.a.c. for the several dominant wave¬ 


lengths. Figure 46 was derived in a similar manner 
from Figure 45. 

The general trends of the results indicated for 


CONFIDENTIAL 









64 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


spots in Figures 43 and 44 are the same as shown 
in Figures 45 and 46 for Landolt rings. The most 
notable exception is indicated by the curves for 



Figure 44. Data from Figure 43 plotted on the I.C.I. 
chromaticity diagram. 


dominant wavelength 612 millimicrons, in Figures 
43 and 45. The failure of the curve in Figure 45 to 
rise as sharply for high purities as the correspond¬ 
ing curve in Figure 43 is attributed to the effects 


of chromatic aberration in the refractive media of 
the observers’ eyes. 

This conclusion was confirmed by the conscious 
reactions of the observers, who reported the red to 
be very prominent, so that the spots were easy to 
find, but “fuzzy,” so that the gap in the Landolt 
ring was detected with difficulty. Similar reports 
were made in the case of blue, blue-purple, and red- 
purple rings of high purity, the last being particu¬ 
larly exasperating on the strong green (G5/8) 
background. Despite the consistency of these re¬ 
ports, all of which were unsolicited and many of 
which w^ere independent, little evidence of the effect 
can be found in the quantitative results. Neverthe¬ 
less, it is concluded that although e.a.c. for colors 
of moderate purity on an achromatic background is 
essentially the same whether determined by the vis¬ 
ibility of spots or the Landolt ring acuity test, chro¬ 
matic aberration in the eye interferes with acuity 
more seriously than with visibility, especially for 
red and blue colors of high purity. 

Combined Contrasts 

Many observations were made with spots and 
Landolt rings differing from their backgrounds in 
luminous reflectance as well as chromaticity. Efforts 
to deduce a general rule for calculating the effec¬ 
tiveness of combined brightness and chromatic con¬ 
trasts were largely frustrated by the frequency of 
erratic and unreproducible data. Figure 47 repre¬ 
sents an effort to test the formula which appears to 


(/) 

< 

tr 


z 

o 

o 


< 

> 

3 

o 

UJ 



Figure 45. Equivalent achromatic contrast of colors approximately equal in brightness to their achromatic sur¬ 
round, based on observations of the orientation of Landolt rings. 


Illuminant: tungsten lamp; color temperature: 3,000 degrees K. 


CONFIDENTIAL 









INFLUENCE OF COLOR CONTRAST ON VISIRILITY 


65 


be most consistent with the data. Although many 
large discrepancies from the formula have been 
noted, these discrepancies have not revealed any 
regularities which might suggest useful modifica¬ 
tions of the formula. This formula is also the 
simplest approximation suggested by current devel- 



Figure 46. Data from Figure 45 plotted on the I.C.I. 
chromaticity diagram. 


opment and theory of the metrics of the color 
domain (MacAdam, 1944). 54 Consequently, al¬ 
though its validity cannot be considered proved, 
and modifications will probably be found necessary 
in the future, the following formula is the best that 
can be given at present: 

C 0 = [C 6 2 + C c 2 ]^. (1) 

In this formula, C 0 is the resultant, equivalent, 
achromatic contrast of a chromatic contrast com¬ 
bined with a brightness contrast, C b is the bright¬ 
ness contrast, and C c is the chromatic component of 
the contrast. The chromatic component (C e ) of the 
contrast should be determined from the most ap¬ 
propriate diagram, Figure 39, 40, 41, or 42, depend¬ 
ing on the background color. Since each background 
color appears to introduce peculiarities into the 
shapes of the contours, no generalization seems per¬ 
missible concerning the shapes of the contours for 
backgrounds appreciably different from those rep¬ 
resented in Figures 39, 40, 41, and 42. In default 


of diagrams for other backgrounds, estimates may 
be based on the present diagrams in cases of neces¬ 
sity, but accuracy should not be expected when 
considerable extrapolation has been employed. 

A few series of observations have been performed 
in order to test the formula for the resultant equiva¬ 
lent achromatic contrast of colors seen against chro¬ 
matic backgrounds. The results were not very accu¬ 
rate but were not inconsistent with the formula, 
which appears to be satisfactory for estimates and 
approximations. 

Constancy of E.A.C. at Various Adaptation 
Levels 

Experimental Procedure. Four gray and four 
chromatic papers were selected which, in the form 
of Landolt rings (gap 0.7 minute) on a neutral 
(N5/), gave scores between 30 and 80 per cent for 
four of the five observers for both 35 foot-lamberts 
of artificial daylight quality and 26 foot-lamberts 
of 3000° K color temperature. Landolt rings were 



Figure 47. A study of combined chromatic and lumi¬ 
nance contrasts. Curve represents equation (1). 

Vertical lines represent the spread of the data. Target: neu¬ 
tral Landolt ring on “sky blue” background (Munsell: PB %). 
Brightness contrast: 10 per cent. 

cut from these same papers, with gaps subtending 
0.7, 1.0, 1.4, 2.8, and 5.6 minutes. Using the 0.7- 
minute gap size for control tests during the same 
sessions, observations were made with the 1.0- 
minute gap for several reduced levels of illumi¬ 
nation until a brightness level was found for which 
the gray rings (1.0-minute gaps) were reported 


CONFIDENTIAL 



















66 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


correctly about as often as were the smaller rings 
(0.7-minute gap) of the same papers at the higher 
levels. This procedure was followed for each of the 
available ring sizes. 

Results. The equivalent achromatic contrasts of 
the colors were found to be the same for all bright¬ 
ness levels between 0.01 and 26 foot-lamberts. Since 
there are no essential differences between the results 
with daylight and ordinary, artificial light, the con¬ 
stancy of e.a.c. is not attributable to the Purkinje 
effect. 

Implications. The primary purpose of the test of 
the constancy of e.a.c. with adaptation level was to 
extend the applicability of the diagrams (Figures 
39 to 41) to all photopic levels (above 1 foot- 
lambert). This purpose was fully accomplished by 
the test. The failure to find any evidence for de¬ 
crease of e.a.c. for levels definitely below photopic 
is surprising and indicates that the disappearance 
of color sensation at low levels may not imply a 
radical change of function but merely a decrease of 
differential sensitivity similar to, and apparently 
proportional to, the decrease of sensitivity to bright¬ 
ness differences. 

Brightness differences are appreciable at very 
low levels only because there is no limit to the 
corresponding contrasts. Color difference, the appre¬ 
ciation of which is implicitly required for the rec¬ 
ognition of any color as distinct from neutral, is, 
on the other hand, fundamentally limited, and for 
levels at which the equivalent achromatic contrast 
cannot be appreciated it is reasonable to expect that 
a color will be indistinguishable from neutral. 
Therefore, no extraneous explanatory principle or 
hypothetical duality of retinal function is. required 
for the explanation of the disappearance of color 
sensation at low levels. The persistence of the sen¬ 
sation of red at levels for which all other colors 
are indistinguishable from gray can be attributed 
directly to the equivalent achromatic contrast of 
red on gray which, for all levels of adaptation, is 
far greater than for any other color. 

Observation Room 

Observations were conducted in a room 30 feet 
long, 15 feet wide, 8 feet high, consisting of sheet- 
rock on a temporary wood frame. The entire interior 
—walls, ceiling, and floor—was sprayed with flat 
white paint. The appearance of the room from the 
observers’ stations is shown in Figures 48 and 49. 


Lighting Arrangements. The room was lighted 
with lamps placed in two banks on each of the side 
walls, 10 and 20 feet from the front wall. The sock¬ 
ets for the lamps were mounted in sheet-metal ducts 
which were connected to an exhaust fan. The lamps 



Figure 48. Observation room as seen from observers’ 
seats. Landolt ring appears on dark background. Shut¬ 
ters are open. 


extended into the room through 3-inch diameter 
holes cut in the sheetrock, which produced a draft 
around the necks of the lamps and facilitated the 
removal of the air heated by contact with the lamps. 
Seven lamps could be mounted in each of the four 



Figure 49. Troffer lighting-units as seen from ob¬ 
servers’ seats. 


banks. Lamps from 15 to 1,000 tvatts were used. 
Separate switches were provided for each lamp, so 
that they could be operated in any and all combi¬ 
nations. For very low levels of illumination, 15-watt 
lamps were located deep in the ventilating ducts 
and the holes in the wall were reduced by opaque 


CONFIDENTIAL 









INFLUENCE OF COLOR CONTRAST ON VISIBILITY 


67 


covers to as small as 1 inch. Fragments of cobalt- 
blue glass from broken 1,000-watt GE Mazda Photo 
Blue bulbs were also placed in the small holes to 
obtain the same quality of light at extremely low 
levels as at high levels. Two types of lamps and the 
arrangement for low-level tests are shown in Figure 
50, which also shows the arrangement of the baffles 
which concealed the lamps from the observers and 
increased the diffusion of the light in the room. 



Figure 50. Interior view of troffer unit. 


A ramp separated the observers’ stations from the 
remainder of the room as shown in Figure 51. The 
observers normally removed their shoes before 
entering the room and wore cotton boot-socks which 
were laundered regularly to prevent the floor from 
getting dirty. The entire room was repainted on 
three different occasions when the floor became ap¬ 
preciably darker than the walls and ceiling. When 
freshly painted, the front wall, including the two 
diagonal panels in the front corners, had less than 
3 per cent variation of brightness. The center of the 
ceiling and floor, from the front wall to 15 feet 


from the front wall, was also uniform and equal in 
brightness to the front wall. The brightness of the 
side walls, and the floor and ceiling near the walls, 
varied as shown in Figures 48 and 49, but no varia¬ 
tion greater than 50 per cent from the brightness of 
the front wall was discovered forward of the edge of 
the baffles which hid the lamp banks nearest the 
observers. 

Presentation of Test Pattern 

The test pattern consisted of Munsell colored 
papers cut with punch and die and mounted with 
tacky rubber cement on paper-covered svnthane 
disks which could be removed and interchanged 
rapidly in the center of a steel plate. This steel 
plate was also covered with a Munsell paper to pro¬ 
vide the surrounding color. Figure 52 shows the rear 
view of this apparatus and several of the synthane 
disks covered with a Munsell light gray paper. The 
target shown in the instrument in Figures 48, 49, 
and 53 is mounted on a Munsell medium gray 
paper, which is also shown covering the large steel 
plate. A flat ring which covers the edge of the steel 
plate to hide any imperfections in the plate or the 
edge of the cover paper is painted light gray (about 
30 per cent reflectance) and can be seen in Figures 
48, 49, and 53. This shade of gray reduces the severe 
contrast between the cover of the steel plate and the 
white wall of the observation room. The fanlike 
shutters, which can be seen in Figures 48 and 49 and 
partly in Figure 53, are painted with this same light 
gray to reduce after-image effects. 

The-steel plate, which carries the large surround¬ 
ing paper and supports the synthane disk, is 
mounted in a ball bearing 12 inches in diameter 
which is attached to the hinged rectangle shown in 
Figures 52 and 53. A self-starting 75-rpm synchro¬ 
nous motor is also mounted on the rectangle and 
rotates the steel plate by means of a belt and pul¬ 
ley. Around the circumference of the bearing are 
mounted eight microswitches, which can be seen in 
Figure 52. These are normally closed-circuit but 
open-circuit when a short cam surface (which can 
be seen under the roller of the left switch in Figure 
52), attached to the rotating plate, actuates the 
switches. The power to the 75-rpm motor passes 
through one of the eight switches, which is selected 
by an 8-point switch, the knob of which is shown 
set into the shelf below the apparatus in Figures 
52 and 53. A cam, mounted on the same shaft as this 


CONFIDENTIAL 








68 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 



Figure 51. Observers at their stations in the observation room. 


commutator, opens the fanlike shutters symmetri¬ 
cally through the action of the roller and gears 
shown in the upper center of Figures 52 and 53. 
The cam and commutator are continuously rotated, 
one revolution every 20 seconds. The shutters are 
held open for 10 seconds and remain fully closed 
for 5 seconds. Power is supplied to the lower motor, 
through the commutator and microswitches, only 
during the 5-second period while the shutters are 
fully closed. This is sufficient for one complete revo¬ 
lution of the steel plate, which begins to rotate as 
soon as the shutters are completely closed and stops 
when the selected microswitch is opened. The posi¬ 
tion at which the plate stops is, therefore, deter¬ 
mined by the position to which the selector switch 
has previously been placed. 

This switch is operated manually according to 
a prearranged schedule but is changed only when 
the shutters are opened, that is, when no power is 
available to drive the plate. The neon-discharge 
lamp, mounted just behind the selector switch, 
glows as long as the shutters are closed. The oper¬ 
ator refrains from changing the selector switch dur¬ 
ing this period. When the random schedule calls 


for the same location of the plate twice in succes¬ 
sion, the power from the commutator is fed to an 
already open microswitch and the plate remains 
stationary. Since the commutator supplies power 
for rotation of the plate only while the shutters are 
closed, the observers never see the pattern in mo¬ 
tion, but it is exhibited every 20 seconds for a full 
10-second observation. The synthane disks, carry¬ 
ing patterns of various shapes, sizes, and colors, can 
be changed while the steel plate is rotating wdthout 
interrupting the periodicity of presentations. For 
convenience, each synthane disk was left in place 
for 10 successive presentations at random orien¬ 
tations. 

The selector switch is combined with a punch 
which cuts a small hole in a strip of 16-mm, un¬ 
processed, cine positive film and advances the film 
one frame for each operation of the switch. The 
orientation of the switch is indicated by the location 
of the hole within the frame of the film. Errors of 
reading the pre-arranged schedule are indicated by 
this device, and the punched film record is used as 
a master against which similar punched film rec¬ 
ords of the responses of the observers are compared 


CONFIDENTIAL 



























INFLUENCE OF COLOR CONTRAST ON VISIBILITY 


69 



A recording punch of the type shown in Figure 54 
was provided for each observer. 

Landolt rings were cut out of the Munsell papers 
with a punch press. Die sets were procured to cut 
rings having the following dimensions. 

Outside diameter 2.8 in. 1.40 0.70 0.50 0.35 0.25 

Inside diameter 1.68 0.84 0.42 0.30 0.21 0.15 

Width of gap 0.56 0.28 0.14 0.10 0.07 0.05 

Subtense of gap (at 30 

feet) 5.6 min. 2.8 1.4 1.0 0.7 0.5 


Figure 53. Target presentation apparatus with hinged 
test-plate lowered. 


Figure 52. Rear view of the apparatus used to pre¬ 
sent the targets. 


A small punch was made for cutting small circular 
disks from Munsell papers, with the diameters 
0.090, 0.075, and 0.060 inch, subtending at 30 feet 
0.9, 0.75, and 0.6 minute respectively. Munsell pa¬ 
pers were coated with a tacky rubber cement before 
the rings or disks were punched out. The rings and 
disks could be stored in a notebook and removed 
by peeling from the page. They could then be 
mounted on the paper-covered synthane disks by 
simple pressure (with a clean pressure pad and 
avoiding abrasion). The rubber cement remained 


Figure 54. Recording punch at observer’s seat. Rec¬ 
ord consisted of holes in 16-millimeter movie film. 

tacky and adhesive after several dozen transfers. 
Razor blades were used to peel the ring or disk 
from the surface to which it was attached. Cotton 


CONFIDENTIAL 












70 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


gloves and tweezers were used in handling larger 
pieces of Munsell papers. 

The 1.4-inch diameter Landolt ring shown in 
Figures 48, 49, and 53 is much larger than necessary 
for successful response but was installed so that the 
pattern could be seen clearly in the photographs. 
With such a great contrast as this, the smallest 
ring (0.25-inch diameter) would be seen correctly 
in a significant fraction of the observations by 
practiced observers. 

Munsell papers were used because of the predomi¬ 
nant role of brightness contrast in visual acuity. 
Numerous sets of Munsell papers are available 
which have approximately the same luminous re¬ 
flectance in daylight and in the artificial daylight 
employed in the experiments. The use of such sets 
makes possible the separate study of the influences 
of chromatic and brightness contrasts preparatory 
to study of the effectiveness of chromatic contrasts 
unrestricted to brightness equality. 

3 ‘ 3 ' 4 Procedure 

For convenience of notation and record, the eight 
possible orientations of the test pattern were num¬ 
bered consecutively, clockwise as viewed by the ob¬ 
servers, beginning with one at the top. The observers 
made no use of these numbers and were required 
merely to turn indicators on their recording punches 
to positions corresponding to their impressions of 
the location of the gap in the ring (or the solid dot 
in visibility tests). A schedule was prepared in ad¬ 
vance of each test period, listing the target arrange¬ 
ments, sequence and repetition of the tests, and the 
random orientations of the pattern, designated nu¬ 
merically. As many as 12 different patterns were 
used in one test session. Each was exhibited 10 suc¬ 
cessive times, in random orientations, and the entire 
schedule was repeated, often with reversed or other¬ 
wise changed order of presentation of the various 
patterns. Patterns consisting of various contrasts of 
gray on gray were usually included among the color 
contrast patterns to check on the level of perform¬ 
ance of the observers and to accumulate a sufficient 
mass of data on achromatic contrasts in order to fur¬ 
nish satisfactory psychometric curves to be used as 
the basis for interpretation of the data for color con¬ 
trasts. 

With the shutters closed, the observers settled in 
their seats in the observing room. The first disk was 
placed in the apparatus and the selector switch and 
master punch was placed at the first number appear¬ 


ing in the schedule, opposite the number of the disk. 
When the power switch was closed, the plate and 
pattern rotated to and stopped at the selected orien¬ 
tation, and shortly thereafter the shutters opened, 
exhibiting the pattern to the observers. As soon as 
the shutters opened, the operator changed the selec¬ 
tor switch to the position indicated by the next 
number in the schedule opposite the number of the 
disk being exhibited. The discharge in the neon 
warning lamp ceased as soon as the shutter opened, 
indicating to the operator the proper time for change 
of the selector switch. Ten seconds after the shutters 
reached their maximum opening they began to close. 
As soon as they were fully closed, a buzzer sounded 
briefly in the observation room, the neon warning 
lamp glowed, and the disk carrying the pattern ro¬ 
tated to the new position. At the sound of the buzzer, 
the observers recorded their judgment of the obser¬ 
vation just completed. They were at liberty to turn 
the indicators of their recorders at any time during 
the observation period and to change their indica¬ 
tions to correspond with revised judgment as much 
as they cared. Only their final judgment was re¬ 
corded at the sound of the buzzer, each operator 
deliberately pressing on the dial of her recorder. The 
operator waited before changing the selector switch 
again until the neon lamp was extinguished, indi¬ 
cating that the shutters were open again and the 
motor circuit was dead. As soon as the switch was 
set to the indicated location, the operator checked 
off each digit in the schedule to avoid duplications 
and omissions. 

When all the digits in a row T had been checked off, 
the selector switch was next set to the position indi¬ 
cated by the first number of the next row. When the 
shutters subsequently closed, the first synthane plate 
was removed from the recess in the steel plate, and 
the one listed next on the schedule was inserted. 
This change could be made during the 5 seconds in 
which the shutters were closed without interrupting 
the rhythm of the observations or interfering with 
the functioning of the rotating mechanism. In this 
way, as many as 12 patterns were exhibited without 
interruption 10 times each, in random but recorded 
orientations, in a period of 40 minutes. During this 
period, the observers operated their recorders once 
for each observation, and the complete record of the 
120 judgments of each observer was contained on a 
strip of 16-mm film, 36 inches long. 

Each pattern was shown 20 or 30 times, the sec¬ 
ond and third repetitions of the complete schedule 
following rest periods of 15 minutes. Many patterns 


CONFIDENTIAL 



INFLUENCE OF COLOR CONTRAST ON VISIBILITY 


71 


were repeated on several days, for various reasons, 
and certain achromatic contrasts were repeated 
every session for weeks as common denominators for 
all of the tests with chromatic contrast. 

When each schedule was completed the master 
record was compared with the schedule. Division 
lines were drawn between the tape records of the 
groups made up of 10 positions of each pattern. 
The notation of each pattern was transcribed on the 
corresponding portion of the master tape, and the 
tape record made by each observer was compared 
with the master. 

3 * 3 * 5 Observers 

Difficulty was experienced in obtaining suitable 
observers for this program. More or less extensive 
data were obtained with 27 individuals, all of whom 
had normal color vision, as indicated by the Ishi- 
hara test. Of the 14 whose data are sufficiently com¬ 
plete to warrant inclusion in our results, three wore 
glasses continually while observing. Since the objec¬ 
tive was to evaluate only the relative contributions 
of chromatic and brightness contrast to acuity, ob¬ 
servers with glasses were tolerated. 

Group I consisted of five high school students, 
ages 17 to 19, who served after school hours and on 
Saturdays and holidays. Special check tests were 
devised and used to make sure that the results were 
not affected by communication of test information 
among observers. 

Group II consisted of five employees of the 
Tiffany Foundation who had been trained and used 
there in similar tests already described. The maxi¬ 
mum age of these observers was 25 years; all were 
recent college graduates, four majors in psychology 
and one major in history, but none had any appre¬ 
ciable preparation in any of the physical sciences. 
This group of observers was far superior to any 
others used in this program. 

This superiority was not in degree of acuity, but 
in stability, reproducibility, and immunity to bore¬ 
dom and the varieties of misbehavior which all of 
the other groups exhibited occasionally. This is at¬ 
tributed to the intellectual curiosity and academic 
background of the girls, who were interested in the 
technique and day-to-day trends of the results. 
Judging from the experience gained in this project, 
there is no adequate substitute for this background 
and attitude in extensive tests of visual acuity. 

The last four observers (Group III) used to com¬ 
plete the program were also recent college or junior 


college graduates, maximum age 25 years. Although 
similar in background to Group II, this group was 
much less stable, probably because of distraction of 
interest and energies by after-hours activities. 

3 ‘ 3 ’ 6 Summary of Results 

Conclusion 1. For moderate achromatic and chro¬ 
matic contrasts, acuity appears to depend on con¬ 
trast in the same manner as visibility. Therefore, 
data obtained with test objects convenient for ex¬ 
perimentation (such as the Landolt ring, employed 
in most of the present investigation) may be ap¬ 
plied to other shapes by the determination of em¬ 
pirical conversion factors, using for these tests any 
convenient but definitely specified contrasts of ob¬ 
ject against background. If these conversion factors 
are determined by field tests, their application ren¬ 
ders the laboratory data useful under those field 
conditions. Fundamental data of such intricate 
phenomena as are the subject of this report cannot 
be obtained by actual field experiments because of 
uncontrollable variations of essential conditions, 
distraction of attention, and interruptions and 
delays due to weather and unfavorable circum¬ 
stances. 

Conclusion 2. For high, increasing chromatic con¬ 
trasts, acuity appears to increase less rapidly than 
visibility. This is believed to be a consequence of 
chromatic aberration in the eye but is not likely to 
be of importance in long-distance observations, since 
even the colors of highly chromatic signal flags are 
considerably desaturated by atmospheric haze. The 
magnitude of this desaturation can be computed for 
any specified set of conditions. The resultant acuity 
will depend on the decreased contrast, but the con¬ 
tribution of the decreased chromatic contrast can 
usually be estimated from the attached diagrams 
because the chromatic contrast is less than the 
limiting value above which the chromatic aberration 
of the eye becomes important. 

Conclusion 3. The colors of figures yielding con¬ 
stant acuity against an equally bright background 
of approximately daylight quality are represented 
by a smooth closed curve around the point repre¬ 
senting the color of the background in the standard 
diagram for representing colors. Such curves are 
presented for colors giving the same acuities as 
achromatic brightness contrasts of 5, 10, 15, 20, and 
25 per cent. 

Conclusion 4- For combined chromatic and bright¬ 
ness contrasts, the acuity appears to be approxi- 


CONFIDENTIAL 



72 


PERCEPTUAL CAPACITY OF THE HUMAN OBSERVER 


mately equal to that for an achromatic contrast 
given by equation (1), 

Co= (C* 2 + C c 2 )*, 

where C b is the brightness contrast, and C c is the 
achromatic brightness contrast equivalent to the 
chromatic component of the contrast, as determined 
by interpolation in the diagrams. The chromatic 
components of contrasts encountered under field 
conditions are rarely over 25 per cent. On the other 
hand, appreciable color contrasts are almost invari¬ 
ably combined with brightness contrasts greater 
than 25 per cent. Therefore, the visibility of objects 
and acuity for identification of detail are primarily 
dependent on brightness contrast under most field 
conditions. When brightness contrasts are limited, 
as in the case of signal flags or panels, color dif¬ 
ferences may increase acuity and visibility, if 
chromaticity contrast can be introduced without 
equivalent sacrifice of brightness contrasts. The dia¬ 
grams given for several different background colors 
may be employed in conjunction with the formula 
given in the first sentence of this paragraph for 
design and estimation of the effectiveness of any 
specified color combinations under any specified con¬ 
ditions. 

Conclusion 5. The effectiveness of chromatic con¬ 
trast in acuity and visibility appears to be pro¬ 
portional to the effectiveness of brightness contrast 
when brightness decreases from 100 foot-lamberts to 
10 -2 foot-lambert. As the adaptation level decreases, 
greater and greater visual angles and contrasts are 
necessary to make any perception of the contrast 
possible. Sufficient increase of brightness contrast is 
always possible, but if the contrast is purely chro¬ 
matic, with brightness contrast excluded, there exists 
a level of adaptation below which the perception of 
contrast is impossible even for large visual angles, 
because chromatic contrasts are fundamentally lim¬ 
ited. If exceedingly severe glare is encountered, in 
which brightness contrasts of less than 30 per cent 
cannot be perceived, it may be expected that chro¬ 
matic contrasts (the effectiveness of which cannot 
exceed some such limit) will not be perceptible and 
otherwise highly chromatic stimuli, such as green, 
blue, or yellow signals, may not be distinguishable 
from white and may be reported as colorless. 
Such conditions have not been tested experimen¬ 
tally. 

Conclusion 6. At or near the limit of visibility, the 
hues of chromatic targets are not perceptible, even 
though the object may be seen in an appreciable 


fraction of the number of observations. This is par¬ 
ticularly true of violet, blue, green, and yellow stim¬ 
uli. Orange and red-purple, as well as red, appear 
reddish or brownish under these circumstances. 

Conclusion 7. When responses are forced for every 
observation, significant percentages of correct re¬ 
sponses are recorded for nearly visible targets even 
when the observer is firmly convinced that the ob¬ 
ject or detail is hopelessly invisible. 

34 PERCEPTION WITHOUT AWARENESS 

The statements of Conclusion 7 are true both for 
spots and for gaps of Landolt rings, for achromatic 
and chromatic contrasts alike, and for all observers. 
Lythgoe 88 reported the same phenomena. He 
wrote: 

In general, the subject is unaware of the sort of results 
he is getting. At the flat portions of the top of the curve he 
finds the task none too easy: it is quite an effort to read the 
test object. At the bottom of the curve, he has no idea that 
he is getting any right answers at all and yet he is getting 
more than the expected value due to guesswork. Inexperi¬ 
enced subjects find it difficult to force themselves to give 
an answer at each exposure of the test object. The remark¬ 
able fact is that with very small test objects when the sub¬ 
ject is under the impression that he is guessing, actually he 
is returning more than one in eight correct answers. In one 
series of experiments, we made the size of the test object 
very small indeed and in this case the number of (correct) 
answers returned was not significantly greater than would 
be expected from pure guesses. 

In another part of his report, Lythgoe describes a 
quantitative test of this phenomenon and writes: 

It is as though the eye and the subject’s answer formed 
part of a purely physical process, the readings getting worse 
as the conditions are made more difficult. Existing at the 
same time is a spectator of the process—the subject’s 
awareness. In our experiments, the subject was invited to 
express his views on the accuracy of the working of the 
physical process. Judged by objective standards, his aware¬ 
ness saw little of the game. When the subjects were quite 
certain they were wrong, actually they were giving thirty 
;per cent correct answers. 

Experience in the present investigation has borne 
this out in very striking fashion. In all cases when 
liminal results (56 per cent correct) were being 
obtained, the observers experienced such difficulty 
that they were almost always unconscious of seeing 
the gap or spot. The rare perceptions of the orienta¬ 
tion of the target were so fleeting and nonrepeatable 
that verification was impossible, and there was no 
alternative to indicating the orientation thus so 
vaguely perceived. In the remaining cases, the ob- 


CONFIDENTIAL 





PRACTICAL VISIBILITY PROBLEMS 


73 


servers had no awareness of seeing any persistent 
irregularity which might indicate the proper orien¬ 
tation, though they looked as carefully and as long as 
they could. The indications seemed to be pure 
guesses, literally forced by the requirements of the 
mechanisms, yet when they followed conscientious 
observation they were correct more often than can 
be accounted for by chance. 

3,4,1 Lookout Procedure 

Since the phenomenon of perception without 
awareness is not dependent on the particular form 
or color of the test object or on the procedure of 
the experiment, it may be of general occurrence. 
Because of its possible bearing on the procedures 
employed by lookouts, this point was emphasized 
to the Army-Navy-OSRD [ANOSRD] Vision Com¬ 
mittee at one of its first meetings. It is evident from 
the data that if use is made of the phenomenon 
described above distant objects may be detectable 
and identifiable at distances as much as 50 per cent 
greater than at present (see Section 4.4.1). This 
conclusion is based on the assumption that the ob¬ 
server is required to be certain before he reports or 
identifies a strange object. In practice, an approach¬ 
ing plane might be reported 50 per cent sooner and 
identification of type may be possible at 50 per cent 
greater range than at present, reducing the hazards 
of destroying friendly planes or of permitting hos¬ 
tile planes to approach dangerously close. Also, 
search for survivors may be subject to improvement 
corresponding to as much as 50 per cent increase of 
radius of visibility and appreciable increase of the 
efficiency of searching within the present limits of 
visibility. 

A Suggested Technique 

It was suggested to the ANOSRD Vision Com¬ 
mittee that such improvements might be obtained 
by providing at least three observers to scan the 
same sector simultaneously. These lookouts prob¬ 
ably should not collaborate, but each might report 
immediately (possibly by some remote indicator 
device) to a central station his estimate of the azi¬ 
muth, range (and altitude of aircraft), and charac¬ 
ter (that is, ship or plane) of each thing that he 
thinks he saw, however fleeting or vague and unveri¬ 
fied the impression may have been. The central 
station might relay such reports to higher authorities 
only if two or more corroborating reports are re¬ 
ceived. This procedure would eliminate almost all 


false reports, but a few might be tolerated in ex¬ 
change for the advantage of 50 per cent or greater 
increase of range of vision. Of course, every lookout 
might also be furnished with the present means of 
reporting immediately any object which he sees or 
identifies positively. Positive reports could be dis¬ 
tinguished clearly from liminal reports and relayed 
(or perhaps transmitted from the lookout by more 
direct channels), even in the improbable event that 
they are not immediately corroborated. 

Thorough trial and modification by careful ex¬ 
perimentation would undoubtedly be necessary to 
perfect a technique of this general character, but the 
probable advantages may be sufficiently important 
to justify the effort. This method may not be feas¬ 
ible on any except large units, because the present 
number of lookouts would probably need to be 
tripled to provide coverage without widening the 
sector of any one lookout. The change of attitude 
required to report all “hunches” immediately and as 
accurately as possible without waiting for self-con¬ 
firmation may be so great that many experienced 
lookouts may be unsuitable and the training of new 
personnel for such duty may have to be considered. 
Such training would cultivate a very attentive, care¬ 
ful attitude completely free of inhibitions regarding 
false alarms. The essence of the method is in the 
fact that when an observer concentrates and reports 
all possibly significant impressions a considerable 
fraction of his “false alarms” prove to have founda¬ 
tion, and when confirmed by an independent report 
of another similar observer the probability of cor¬ 
rectness approaches certainty. A triple coincidence 
should be more reliable than a “positive” report by a 
single observer and may extend the range of visi¬ 
bility and time available for countermeasures 50 per 
cent or more. 

No word has been received that the suggested 
technique was tried by Navy lookouts, but it is 
understood that recent editions of the Manual for 
Lookout Instructors advocate the reporting of 
“hunches.” 

*•* PRACTICAL VISIBILITY PROBLEMS 

Before the results of the extensive experimental 
programs described in this chapter can be used in 
the solution of practical visibility problems, means 
must be provided for combining these data with 
information regarding the optical state of the at¬ 
mosphere. Nomographic charts for this purpose are 
presented in the two following chapters. 


CONFIDENTIAL 



Chapter 4 

THE VISIBILITY OF NAVAL TARGETS 


INTRODUCTION 

T he limiting range at which a ship at sea, a low- 
flying aircraft, or a shoreline may be sighted 
can be predicted from the principles and data which 
have been presented in the two preceding chapters. 
Such a prediction, however, requires tedious com¬ 
putations, impractical under operational conditions. 
It is the purpose of this chapter to show how the 
computations can be avoided by the use of simple 
nomographic charts and to illustrate the use of such 
charts in predicting the limiting range at which any 
specified target will be just visible. 


Illustrative Example 

Let it be required to find the distance at which a 
uniform circular target having a projected area of 
100 square feet and a brightness of 10 foot-lamberts 
will be liminally visible on a day when the meteoro¬ 
logical range is 20,000 yards, assuming the target to 
be viewed along a homogeneous, horizontal path 
against a uniform background of horizon sky, the 
brightness of which is 1,000 foot-lamberts. The in¬ 
herent contrast of the target is 


C 


0 


10 - 1000 
1000 


0.990. 


42 THE NATURE OF THE PROBLEM 

Previous chapters have shown that the visibility 
of a uniform target depends upon the apparent con¬ 
trast between the target and its background, the 
angular size of the target, its shape, and the percep¬ 
tual capacity of the observer at the level of bright¬ 
ness to which his eyes are adapted. Both the appar¬ 
ent contrast and the angular size of a target vary 
with target distance, but in accordance with different 
laws. For example, at a distance of X yards, a cir¬ 
cular target of area A square feet subtends an angle 
a given by 

1293 V A . . , 

a —-- -- minutes of arc. (1) 

The apparent contrast C x of any target at dis¬ 
tance X is related to its inherent contrast C 0 by the 
relation 

C x = C 0 e~ 3912X/v , (2) 

where v is the meteorological range. Because the 
perceptual capacity of a human observer depends 
simultaneously upon both a and C x , in the manner 
shown by Figure 35, Chapter 3, any calculation in¬ 
tended to determine the range at which a target can 
just be sighted must consist of a series of successive 
approximations. In other words, the answer must 
be found by bracketing. The procedure is illustrated 
by the following example. 


This value indicates that, to an observer close 
aboard, the target appears as a nearly black sil¬ 
houette. 

Since the meteorological range is 20,000 yards, it 
may be assumed that a very large black object 
would be liminally visible at approximately that 
range. However, at 20,000 yards the angle subtended 
by the target is shown by equation (1) to be only 
0.646 minutes. Referring to Figure 35, Chapter 3, 
or Appendix A, the liminal contrast for a target of 
this angular size is — 0.355. However, from the 
definition of meteorological range, the apparent con¬ 
trast of the target is — 0.020. Hence, the target is 
invisible at 20,000 yards. 

Although the liminal target distance is now known 
to be less than 20,000 yards, its actual value must 
be found by trial and error. Assume the target to be 
at 10,000 yards. At this distance it subtends an 
angle of 1.292 minutes, and its apparent contrast is 
shown by equation (2) to be — 0.145. From Figure 
35, Chapter 3, or Appendix A, the liminal contrast 
for a target of this angular size is — 0.0966. Since 
the magnitude of the apparent contrast exceeds the 
magnitude of the liminal contrast, the target can be 
seen at 10,000 yards and beyond. 

In order to bracket the answer in a systematic 
manner, let equation (2) be used to find the distance 
at which the apparent contrast is — 0.0966. This 
is found to be 11,930 yards. At this distance the 
target subtends an angle of 1.083 minutes, and the 
corresponding liminal contrast is — 0.134. Since this 


74 


CONFIDENTIAL 



LIMINAL TARGET DISTANCE (YARDS) 


THE NOMOGRAPHIC METHOD 


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LIMINAL TARGET DISTANCE (YARDS) 

Figure 1. Nomographic visibility chart. 

Curved line represents data from Figure 35 for uniform, circular target 100 square feet in area when B a = 1,000 foof-lamberts. 
























































































76 


THE VISIBILITY OF NAVAL TARGETS 


exceeds the apparent contrast in magnitude, the 
target is not visible. 

It is evident that the distance at which the target 
is liminally visible has been bracketed. If the brack¬ 
eting process is continued, the answer finally at¬ 
tained is 11,000 yards. 

The foregoing calculation is obviously too cum¬ 
bersome, too time consuming, too subject to error 
to be practical for routine use. Fortunately, the 
calculation can be avoided completely by the use 
of nomographic charts. 

43 THE NOMOGRAPHIC METHOD 

Figure 1 shows a nomographic chart capable of 
making simultaneous allowances for the variations 
of a and C x with target distance. The curved line 
which crosses the center of the figure represents data 
from Figure 35, Chapter 3. Specifically, it represents 
the limiting perceptual capacity of a typical ob¬ 
server whose eyes are adapted to full daytime sky 
brightness when the target is uniform, circular, and 
100 square feet in area. A target of any other area 
could be represented by a different curve, and subse¬ 
quent nomographic charts in this volume contain a 
family of curves corresponding to a billionfold range 
of areas. 

Illustrative Example 

To illustrate the use of the nomographic visibility 
chart, let the example of the preceding section be 
solved by means of Figure 1. Lay a straightedge 
across the chart in such a manner that it connects 
20,000 yards on the meteorological range scale with 
0.99 on the contrast scale. (The dashed line in Fig¬ 
ure 1 indicates the position of the straightedge.) 
From the point where the curve is intersected by the 
straightedge, move straight up or straight down to 
the target distance scale. The answer, 11,000 yards, 
read from this scale, agrees with that previously ob¬ 
tained by bracketing. 

4 31 Special Cases 

Fog 

The scales of meteorological range and liminal 
target distance on the nomographic visibility chart 
shown in Figure 1 may be multiplied by any factor, 
provided the value of area assigned to the curve is 
multiplied by the square of the factor. This conven¬ 


ient property of the charts enables them to be used 
for problems involving small values of meteorologi¬ 
cal range. 

For example, let the scales of meteorological range 
and liminal target distance in Figure 1 be multiplied 
by 1/10, so that the former covers values down to 
400 yards and the numbered divisions of the latter 
begin with 50 yards and end with 50,000. The curved 
line, which formerly applied to a target 100 square 
feet in area, now applies to a target whose area is 1 
square foot. Thus, as indicated by the dashed line, 
a circular target 1 square foot in area and having an 
inherent contrast of ± 0.99 is liminally visible at 
1,100 yards on a day when the meteorological range 
is 2,000 yards. Obviously, if Figure 1 bore a curve 
corresponding to a target area of 10,000 square feet 
on the basis of the scales as originally numbered, 
the curve would apply to a target 100 square feet in 
area when the chart is used in the manner just 
described. 

Targets of Very Large Area 

In dealing with targets of very large area or 
targets visible at very long distances, the range and 
distance scales of the visibility charts may advan¬ 
tageously be multiplied by 10. If this is done in 
Figure 1, the curved line then applies to targets 
10,000 square feet in area, and the dashed line 
indicates that such a target will be liminally visible 
at 110,000 yards on a day when the meteorological 
range is 200,000 yards, provided the inherent con¬ 
trast of the target is dz 0.99. 

Exact Values of Target Area 

Since the factor by which the scales are multiplied 
may have any value, the curved line in Figure 1 
can be made to apply to any area. 

For example, let it be required to find the liminal 
target distance for a target whose area is 64 square 
feet, assuming, as before, that the inherent contrast 
of the target is ± 0.99 and the meteorological range 
is 20,000 yards. Since the area represented by the 
curve must be multiplied by 0.64, the range and 
distance scales are to be multiplied by 0.80. This 
means that the division marked 20,000 yards on the 
meteorological range scale corresponds with a 
meteorological range of 16,000 yards. A meteoro¬ 
logical range of 20,000 yards is therefore represented 
by the division numbered 25,000. If this point is 
connected by a straightedge (not shown) to 0.99 on 
the contrast scale, the intersection of the curve and 


CONFIDENTIAL 






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THE NOMOGRAPHIC METHOD 


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78 


THE VISIBILITY OF NAVAL TARGETS 


the straightedge indicates 12,400 on the scale of 
target distance. After multiplying by the scale fac¬ 
tor 0.80, the liminal target distance is found to be 
9,920 yards. 

The family of curves, each representing a target 
of different area, which appear on all the nomo¬ 
graphic charts presented later in this volume are 
intended to make unnecessary the type of calcula¬ 
tion just described. However, for the construction of 
tables, or for special computations requiring great 
precision, the method described in this section should 
be used. 

4 * 3 2 Other Uses of the Nomograph 

The Determination of Liminal Contrast 

The nomographic visibility chart may be consid¬ 
ered as a special plot of the liminal contrast data 
given in Figure 35, Chapter 3. 

For example, let it be required to find the liminal 
apparent contrast of a target 100 square feet in area 
and 10,000 yards from the observer. Place a straight¬ 
edge across Figure 1 in such a manner that it con¬ 
nects the infinity point at the top of the meteoro¬ 
logical range scale with the intersection of the curve 
and the vertical line representing 10,000 yards. The 
straightedge then intersects the contrast scale at 
zb 0.097, the value of liminal contrast for a target 
of this angular subtense. This implies that all lim- 
inally visible uniform circular targets subtending 
the same angle at the observer’s eye are represented 
by a straight line connecting the infinity point on 
the meteorological range scale with the point repre¬ 
senting the liminal contrast. 

Precise Values of Liminal Contrast. The curved 
line on Figure 1 was constructed by marking the 
point of intersection of each vertical target-distance 
line with a straightedge connecting the infinity point 
on the meteorological range scale with the appro¬ 
priate value of liminal contrast. Before this could be 
done, a table showing the values of liminal contrast 
for each intersection was prepared from very large- 
scale plots of Figure 35, Chapter 3. This table 
gives the values of liminal contrast more precisely 
than they can be read from either Figure 35, Chap¬ 
ter 3, or Figure 1 . Tables of this type for all the 
visibility charts in this volume are given in the 
microfilm supplement , 23b and should be consulted to¬ 
gether with Appendix A whenever new tables or new 
charts are prepared. 


The Determination of Apparent Contrast 

The nomographic visibility chart can be used to 
solve equation (2). For example, let it be required 
to find the apparent contrast of a target of inherent 
contrast zb 0.99 when it is 10,000 yards from the 
observer on a day when the meteorological range is 
20,000 yards. Place a straightedge across Figure 1 
in the position shown by the dotted line. Place the 
point of a pencil at the intersection of the straight¬ 
edge with the vertical target-distance line for 10,000 
yards. Rotate the straightedge until it passes through 
the infinity point at the top of the meteorological 
range scale. The straightedge now intersects the con¬ 
trast scale at zb 0.145, the apparent contrast of the 
target. 

Obviously, this technique can be employed to 
solve for any of the four quantities in equation (2). 

Structure of the Nomograph. A mathematical dis¬ 
cussion of the nomographic charts is included in the 
microfilm supplement. 39 

4 * 3 * 3 Nomographic Charts for Circular 
Targets 

More than a million observations of uniform, cir¬ 
cular targets were made by a homogeneous group of 
observers at the Tiffany Foundation (Chapter 3). 
The observing conditions, covering the entire gamut 
of brightness conditions from the brightest day to 
the darkest night, were carefully controlled and 
accurately measured. No visibility experiment of 
comparable magnitude or thoroughness has ever 
been reported; the Tiffany data are believed to pos¬ 
sess far greater reliability than any other visual 
data. It is appropriate, therefore, to present these 
data in the form of nomographic charts similar to 
Figure 1. A set of such charts is presented in Figures 
2 through 10. These charts cover adaptation bright¬ 
nesses B h from 10“ 5 foot-lambert to 1,000 foot-lam- 
berts in decimal steps. 

Each of the nine figures (Figure 2 through 10) is 
a nomographic visibility chart for uniform circular 
targets seen against a uniform background of hori¬ 
zon sky having the brightness B H indicated at the 
lower right corner of the diagram. Descriptive 
phrases, such as overcast day or quarter moon, have 
been added to serve as a rough guide in selecting the 
proper chart for use in a given problem. 

These charts possess- reliability of a very high 
order when used to predict the distance at which a 


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THE NOMOGRAPHIC METHOD 


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LIMINAL TARGET DISTANCE (YARDS) 


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94 


THE VISIBILITY OF NAVAL TARGETS 


uniform, circular target will be liminally visible 
when viewed along a homogeneous, horizontal sight 
path against a uniform background of horizon sky. 
The use of the visibility charts (and modifications 
of them) for the solution of problems of increased 
complexity and greater realism is described in later 
sections of this chapter. In such cases, the reliability 
of the predicted values of liminal target distance 
depends upon the accuracy with which the conditions 
assumed by the user of the charts agree with the 
actual conditions. 

44 THE SIGNIFICANCE OF LIMINAL 
TARGET DISTANCE 

When used in the manner described in the fore¬ 
going section, the nomographic visibility charts pre¬ 
dict the distance at which the target will be limi¬ 
nally visible. It was explained in Chapter 3 that a 
target is liminally visible when an observer who is 
forced to judge whether the target is present or 
absent is as likely to be right in his judgment as he 
is likely to be wrong, correction having been made 
for chance. Unfortunately, the observer is quite un¬ 
aware that half his answers are right. He has no 
confidence that he has seen the target. The proba¬ 
bility of an observer voluntarily reporting the pres¬ 
ence of a liminally visible target is nearly zero. 

4,41 The Sighting Range 

At some range less than the liminal distance, the 
observer becomes conscious of seeing the target. The 
Tiffany observers became convinced that the thresh¬ 
old of confidence usually coincides with a 90-10 
chance of making a correct report. They also dis¬ 
covered that in terms of contrast the slope of the 
psychometric function is nearly independent of 
adaptation level and target size. The slope is such 
that if a given target is liminally visible , a similar 
target having double the contrast will be seen with 
threshold confidence. 

The approximate range at which a target will be 
seen with threshold confidence can be predicted from 
the nomographic visibility charts by dividing the 
inherent contrast of the target by two before enter¬ 
ing the data on the chart. For example, if, in Figure 
1, the dashed line connected 20,000 yards on the 
meteorological range scale with 0.50 on the contrast 
scale, a target distance of 9,260 yards would be indi¬ 


cated. At approximately this distance, called the 
sighting range, the target would be seen with thresh¬ 
old confidence. Nomographic charts have been 
drawn to indicate the liminal target distance rather 
than the sighting range, because the former quantity 
has a precise physical significance not shared by the 
latter. 

45 VISIBILITY OF NONCIRCULAR 
UNIFORM TARGETS 

One of the first experiments performed by the 
Tiffany Foundation compared the visibility of the 
silhouette of a ship with that of a circular target 
having the same area. The experiment was repeated, 
using the silhouette of an airplane. These experi¬ 
ments suggested that uniform targets of equal area 
and equal apparent contrast are equally visible, re¬ 
gardless of their shape. Later experiments showed, 
however, that in certain extreme cases a correction 
for target shape is required. Indeed, as shown by 
Figure 36, Chapter 3, the liminal contrast of a uni¬ 
form rectangular target, having a length 100 times 
its width and subtending a solid angle of 100 square 
minutes, has been found to be more than six times 
greater than the liminal contrast of a square or cir¬ 
cular target of the same area. The visibility of a 
uniform target of any shape is never greater than 
the visibility of a uniform circular target of the same 
area and apparent contrast. 

Visibility Charts for Rectangular Targets 

Each of the sixteen figures (Figure 11 through 26) 
is a nomographic visibility chart for uniform rec¬ 
tangular targets seen against a uniform background 
of horizon sky having the brightness B H indicated 
at the lower right corner of the diagram. The side- 
to-side ratio of the rectangle to which a chart ap¬ 
plies is indicated at the lower left corner of the 
diagram. 

Because the form factors for rectangles reported 
in Section 3.2.9 depend upon the angular size of the 
target and therefore upon its distance, special visi¬ 
bility charts are required for rectangular targets. 
Such charts are presented in Figures 11 through 26. 
They have been produced by applying the appro¬ 
priate values of form factor to the data from which 
Figures 2 through 10 were plotted. The visibility of 
a rectangular target of side-to-side ratio for which 
no chart is given can be inferred by finding the re- 


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126 


THE VISIBILITY OF NAVAL TARGETS 


spective liminal target distances for a more extreme 
and a less extreme case. For example, the visibility 
of a 7:1 rectangle is intermediate between the visi¬ 
bility of the corresponding 10:1 and 4:1 rectangles 
for which charts are given. The visibility of uniform 
targets of irregular shape is usually identical with 
the visibility of the uniform rectangular target that 
they most closely resemble. This rule does not apply 
to a hollow rectangle or to an annulus. Such a target 
should be treated in the manner described in Sec¬ 
tion 4.9. 

46 BACKGROUNDS OTHER THAN THE SKY 

It was shown in Section 2.3.7 that, in the case of 
targets viewed against backgrounds other than the 
sky, the apparent contrast at distance X of a target 
seen against any background is related to the inher¬ 
ent target contrast by the expression 



where B H /B 0 is the ratio of the brightness of the 
horizon sky in the direction of the target to the 
brightness of the background of the target and v is 
the meteorological range. B H /B 0 is a limiting case 
of the sky-ground ratio discussed in Section 2.3.6. 

4,6,1 Visibility Charts for Any 

Background 

The nomographic visibility chart shown in Figure 
27 is identical with Figure 1 except that a sky- 
ground ratio scale has been added along the inside 
left margin, and the contrast scale has been moved 
to the center of the figure. 

To illustrate the manner of using this chart, let 
the numerical example of Section 4.3 be re-solved for 
the case of a target viewed against a background 
having a brightness of 200 foot-lamberts. Since the 
brightness of the horizon sky was assumed to be 
1,000 foot-lamberts, the sky-ground ratio is 5.0. The 
inherent contrast of the target against its back¬ 
ground is 


Place a straightedge across the chart in such a 
manner as to connect 5.0 on the sky-ground ratio 
scale with =±= 0.95 on the contrast scale. The position 


of the straightedge is shown by the dotted line in 
Figure 27. Place the point of a pencil at the inter¬ 
section of this line with the right-hand vertical 
boundary of the chart. Rotate the straightedge until 
it connects this point with 20,000 yards on the 
meteorological range scale as shown by the dashed 
line. From the intersection of the dashed line and 
the curve, proceed vertically to a reading of 6,900 
yards on the scale of liminal target distance. 

A complete set of charts similar to Figure 27 is 
presented in Chapter 5 for dealing with problems of 
visibility downward along slant paths. Figure 6 
corresponds with Figure 27. Figures 6 to 30 should 
be used for the solution of problems of the type 
illustrated in this section. 

4,6,2 Uncertain Adaptation 

Whenever a target is viewed against a background 
limited in angular extent and differing in brightness 
from the major portion of the field of view, uncer¬ 
tainty exists concerning the effective level of bright¬ 
ness to which the eyes of the observer are adapted. 
When the background of the target appears dark, 
the liminal target distance may be less than would 
be predicted by assuming the observer to be 
adapted to the brightness of the major portion of 
the field of view. 

When the background of the target appears bright, 
the true liminal target distance may exceed the pre¬ 
dicted value. This is illustrated by a ship seen as a 
silhouette against the moon. In this case, a first- 
order correction can be applied by using a nomo¬ 
graphic visibility chart based upon the apparent 
brightness of the moon rather than upon the bright¬ 
ness of the night sky. 

47 THE VISIBILITY OF SIGNAL LIGHTS 

The illumination on the pupil of an observer’s eye 
produced by a distant point source of intensity I 0 
is given by 

Ex = ~e- 3M2X '\ (4) 

where v is the meteorological range. This relation 
is valid only when X is so great that the light may 
be considered a “point source” in the sense that the 
product of target area and liminal contrast is con¬ 
stant. 


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128 


THE VISIBILITY OF NAVAL TARGETS 


4 ' 71 Maximum Angular Size of a 
Point Source 

The curves of Figure 35, Chapter 3, are straight 
lines for small values of angular target size. The 
slope of these straight lines is — %, as would be 
expected from equation (4). Obviously, the maxi¬ 
mum angular size of target for which equation (4) 
is valid is indicated by the point at which the curves 
in Figure 35 of Chapter 3 depart from a straight 
line. Table 1 has been obtained by inspection of a 
large-scale plot of that figure. 


Table 1 


Adaptation brightness 
(foot-lamberts) 

Maximum angular size 
(min of arc) 

1.000 

0.708 

100 

0.708 

10 

0.750 

1 

0.891 

io- 1 

1.30 

io- 2 

2.82 

io- 3 

6.68 

io- 4 

8.55 

io- 5 

15.0 


From this table, the range beyond which equation 
(4) applies to a searchlight of area A can be found 
by solving equation (1). For example, on a night 
when the sky brightness is 10~ 3 foot-lambert, a sig¬ 
nal light having an area of 1 square foot may .be 
considered as a point source beyond 

v 1293VI lnA 
X = ^^= myards - 

4.7.2 ^ Nomographic Visibility Chart 

for Signal Lights 

Figure 28 is a visibility chart for predicting the 
range at which signal lamps or other point sources 
will be liminally visible. The chart is similar to the 
foregoing visibility charts in this volume, except 
that the contrast scale has been replaced by a scale 
of intensity. Each curve represents a decimal value 
of adaptation brightness. 

As an example of a use of this chart, let it be 
required to determine the intensity of a signal lamp 
liminally visible at 10,000 yards on a foggy night 
when the sky brightness is 10~ 3 foot-lambert and the 
meteorological range is 5,000 yards. Place a straight¬ 
edge across the chart so that it connects 5,000 yards 


on the meteorological range scale with the intersec¬ 
tion of the 10,000 yards distance ordinate and the 
curve representing an adaptation level of IO -3 foot- 
lambert. The intersection of the straightedge with 
the intensity scale of the chart indicates the required 
liminal intensity of the signal lamp to be 2,500 
candles. 

18 VISIBILITY THROUGH BINOCULARS 

The distance at which a specified target is limi¬ 
nally visible through perfect binoculars can be 
found from the nomographic visibility charts by 
multiplying the area of the target by the square of 
the magnifying power of the binoculars before en¬ 
tering the data on the chart. For example, suppose 
a pair of perfect 7-power glasses is used by the ob¬ 
server in the example of Section 4.3. Since the area 
of the target is 100 square feet, the area used in 
entering the chart is 4,900 square feet and the re¬ 
sulting liminal target distance is 22,500 yards. The 
liminal target distance for the unaided eye was 
shown in Section 4.3 to be 11,000 yards. It will be 
noted that, although 7-power glasses were used, the 
liminal target distance was increased by but a factor 
of 2. Only when the meteorological range is infinite 
do perfect binoculars having a magnifying power M 
permit objects to be seen M times as far as they 
can be seen by the naked eye. 40 

The foregoing discussion applies to perfect binoc¬ 
ulars, by which is meant an instrument whose only 
effect is to increase the apparent angular size of the 
target. Actually, even the best binoculars fall some¬ 
what short of the ideal, so that the liminal target 
distance predicted by means of the visibility charts 
should be considered as a limiting value, never ex¬ 
ceeded but often approached by observers using real 
binoculars. 

49 THE VISIBILITY OF NONUNIFORM 
TARGETS 

A ship or a plane is usually seen as a nonuniform 
target, because of its complex three-dimensional 
shape. Even if the target is painted uniformly, il¬ 
lumination differences produce a pattern of high¬ 
lights and shadows. Although countershading is 
sometimes employed to lessen the internal contrast 
of the pattern, it is seldom possible to compensate 
fully for the differences in illumination. The dis- 


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130 


THE VISIBILITY OF NAVAL TARGETS 


tance at which a nonuniform target is liminally 
visible can be predicted from nomographic visibility 
charts only if an effective value of inherent contrast 
can be found and only if this value is nearly inde¬ 
pendent of liminal target distance. 

Two approaches were made to this problem. (1) 
Studies were begun by the Tiffany Foundation in¬ 
tended to disclose the basic principles governing the 
relation between the size, shape, and brightness of 
the components of a pattern and its effective con¬ 
trast. (2) The visibility of a photographic model of 
a cruiser (Figure 29) was compared with the visi¬ 
bility of a uniform target of equal projected area. 
Neither of these experiments was completed, but the 
fragmentary results suggest certain practical rules 
which will be summarized in the following sections. 



Figure 29. Photograph of a 20-foot model of a cruiser. 


4,91 The Visibility of Naval Targets in 
Clear Weather 

Nearly all naval targets present patterns charac¬ 
terized by high, inherent, internal contrasts. Under 
most situations their inherent integrated contrasts 
are also high. In very clear weather such a target 
subtends but a small angle when it is liminally vis¬ 
ible. In this case, the liminal target distance can be 
found from the nomographic visibility charts by 
using the inherent integrated contrast as the value 
of effective inherent contrast. 

Under certain circumstances of lighting and ob¬ 
servation, the inherent integrated contrast may ap¬ 
proach zero. When this occurs, the effective inherent 
contrast must have some value substantially greater 
than zero, inasmuch as zero effective contrast im¬ 
plies that the target is invisible regardless of how 
close it may be. During the Tiffany experiments it 
was found that when the inherent integrated con¬ 
trast of the cruiser model was zero, it was liminally 
visible at the same distance as a uniform target of 
the same projected area having an inherent contrast 


of approximately unity. General conclusions based 
on this result are probably unwarranted, but the 
experiment proved that, for the cruiser model tested, 
the effective inherent contrast should be taken as 
equal to the inherent integrated contrast or unity, 
whichever is greater. It is probable that this rule 
applies to most naval targets under most circum¬ 
stances of observation. However, in the case of skill¬ 
fully camouflaged targets viewed under the most 
favorable circumstances, the minimum value of 
effective inherent contrast may be substantially less 
than unity. Experiments with models of all types of 
naval targets should be conducted in a visibility 
theater, in order to determine for each the minimum 
value of effective inherent contrast. 

4,9,2 The Visibility of Naval Targets in 
Foggy Weather 

It remains to be determined whether or not the 
effective inherent contrast of a target having zero 
inherent integrated contrast is independent of limi¬ 
nal target distance. The experiment described in the 
preceding section tested only the clear-weather case. 
Had time permitted before the expiration of the 
Tiffany contract, the experiment would have been 
repeated using a series of photographic models hav¬ 
ing successively lower contrasts, and the liminal 
target distances so obtained would have been com¬ 
pared with liminal target distances predicted by the 
nomographic visibility charts using a fixed value 
(unity) of effective inherent contrast. It is recom¬ 
mended that such an experiment be performed and, 
if agreement is found, no special corrections are 
necessary when the nomographic visibility charts are 
used to predict the visibility of naval targets in 
foggy weather. 

49 3 The Effect of Color 

It was shown in Chapter 3 that the equivalent 
achromatic contrast (C c ) of even the most garish 
color contrast seldom exceeds 0.5, and that the re¬ 
sultant equivalent achromatic contrast (C 0 ) of a 
color contrast combined with a brightness contrast 
(C 6 ) is given by equation (1) of Chapter 2. 

C 0 = (C b 2 + C c 2 )\ 

As shown in Section 4.9.1, the effective inherent 
contrast of a typical naval target is usually unity or 
greater because of the pattern formed by the high- 


CONFIDENTIAL 





THE MEASUREMENT OF CONTRAST 


131 


lights and shadows. If an extreme color contrast is 
added, the effective inherent contrast is increased 
by only 12 per cent: 

C 0 = (1.00 2 + 0.50 2 )^ = 1.12. 

It will be seen from the nomographic visibility 
charts that this increase in contrast produces only a 
small change in liminal target distance. For exam¬ 
ple, on a day when the meteorological range is 20 
miles, a target having an area of 100 square feet and 
a contrast of 1.00 is liminally visible at 14,000 
yards. A similar target having a contrast of 1.12 is 
liminally visible at 14,600 yards. 

Ships and planes are seldom painted highly chro¬ 
matic colors. Ordinarily the maximum color con¬ 
trast encountered in time of war is represented by 
a gray ship seen against a sky-blue background or 
by the reverse, a blue ship seen against a gray back¬ 
ground. Figures 39 and 40, Chapter 3, show that in 
neither case does the equivalent achromatic con¬ 
trast exceed 0.12. The corresponding increase in ef¬ 
fective inherent contrast is only 0.7 per cent: 

C 0 = (1.00 2 + 0.12 2 ) % = 1.007. 

The effect on the liminal target distance of so small 
an increase in contrast is negligible. Ordinarily, the 
color of a naval target does not affect the distance 
at which it is liminally visible. This statement has 
no bearing upon the noticeability of a readily visible 
target. 

410 THE MEASUREMENT OF CONTRAST 

The effective inherent contrast of a ship or a plane 
has been shown to be equal to the inherent inte¬ 
grated contrast of the target until this quantity 
falls below some minimum value which depends 
upon the nature of the target and the lighting con¬ 
ditions. It is necessary, therefore, to provide means 
for measuring the integrated contrast of a target and 
for specifying the nature of the lighting conditions. 

Maxwellian View Photometers 

Clerk Maxwell proposed a photometer in which 
a lens is used to form an image of the target on the 
pupil of the observer’s eye. The lens then appears 
uniformly bright; its brightness, apart from light 
losses in the lens itself, equals the integrated bright¬ 
ness of the target. Most visual photometers can be 
modified for use as Maxwellian view devices. Al¬ 


though their accuracy is affected somewhat by the 
Stiles-Crawford effect, 41 their results are usually 
sufficiently reliable for use in visibility calculations. 

A convenient Maxwellian-type photometer can 
be produced by fastening a short-focus photographic 
objective (/ = 2 inches) to the front end of the 
drawtube of a Luckiesh-Taylor brightness meter. 42 
The quality of the photometric field can be improved 
by cementing a tiny positive lens to the front (inner) 
end of the ocular tube. 

When a Maxwellian view photometer is used for 
the determination of the integrated contrast of a 
ship or plane, allowance must be made for the fact 
that the target does not fill the field of view of the 
photometer precisely. One method for making such 
allowances will be described in the next section. 

410 2 An Integrating Contrast Photometer 
for the Study of Models 

A recording photoelectric photometer for study¬ 
ing the integrated contrast of model ships and model 
planes was built and used by the Tiffany Founda¬ 
tion to measure models of submarines supplied by 
the Bureau of Ships and models of aircraft supplied 
by the Bureau of Aeronautics. 44 Thereafter, the in¬ 
strument was moved to the U. S. Naval Air Station 
at Patuxent River, Maryland, where it is in use by 
Navy personnel. 

Comparative studies of the integrated contrast of 
different camouflage designs can be made very 
quickly with this instrument, the polar curves of 
integrated contrast drawn by the photometer indi¬ 
cating the directions in which the target is least 
likely to be visible. 

The High Hill Project 

In order to provide the photometer with an unob¬ 
structed view of the horizon,"the Tiffany Foundation 
erected a 50-foot tower atop High Hill, South Hunt¬ 
ington, Long Island. This site, only a short distance 
from the Tiffany estate, is the highest point on 
Long Island. The tower, shown in Figure 30, 
elevated the apparatus above the tree tops. The 
photometer was mounted on the roof of the inclosure 
which housed the recording apparatus at the top of 
the tower. 

Figure 31 shows the photometer assembly before it 
was mounted in the tower. The model C is mounted 
on the outer end of an 8-foot arm E, designed to be 
rotated by the vertical shaft F. Two identical photo- 


CONFIDENTIAL 



132 


THE VISIBILITY OF NAVAL TARGETS 




Figure 31. Integrating contrast photometer before 
being mounted in the tower. 


Figure 30. Tower on High Hill, South Huntington. 
Long Island. 

From its top the integrating contrast photometer had an un¬ 
obstructed view of the horizon. A model of a submarine is in 
place at the end of the photometer arm. 

electric photometers, A and B , are supported and 
rotated by the same shaft. The field of view of the 
lower photometer B includes the model, but that of 
the upper photometer A does not. A vacuum-tube 
bridge circuit ( Figure 32), housed in box D, is used 
to compare the photocell currents from the two 
photometers. A shutter in photometer A, operated 
by control J, adjusts the amount of light entering 
the top photometer until it equals the light entering 
photometer B. This condition is indicated by a zero 
(center) reading of the light-beam galvanometer R 
on the operator’s table. The setting of the shutter is 
indicated by recording pen P on polar graph paper 
attached to turntable H, which is geared to shaft F. 
The photometers, model, and graph paper are ro- 


C 

_T 

-—E 


w 
8 1 
I i- • 


SHIELD 


lOK-n. jo 

I75K A iGALVl ^75KA 


Figure 32. Circuit diagram of the integrating con¬ 
trast photometer. 


Figure 33. Control desk and recording mechanism of 
the integrating contrast photometer. 

tated simultaneously by crank G. A close-up view 
of the control desk and recording mechanism is 
shown in Figure 33. 


CONFIDENTIAL 




































































THE MEASUREMENT OF CONTRAST 


133 



Figure 34. Photographs of typical models tested with the integrating contrast photometer. 

Metal masks of the type shown below each photograph were .used to limit the fields of view of the photometers. 


Correction for Extraneous Background 

The fields of view of both photometers are con¬ 
trolled by identical metal masks cut to match the 
shape of the targets as closely as possible. Two such 
masks are shown in Figure 34. The areas of the 
masks and of the image of the model are determined 
by exposing a photographic plate behind the mask 
and measuring the areas on the resulting picture. A 
correction for the extraneous background surround¬ 
ing the image is made in the following manner. 

Let K — the ratio of the light received by pho¬ 
tometer B to the light received by pho¬ 
tometer A ; 

B h =the brightness of the horizon sky; 

B m — the average brightness of the model; 

A s — the area of the image of the field stop in 
the plane of the model; 

— the projected area of the model. 


Then 


tt _ B m A m -j- (As — A y) Bh 

A 8 B h 

The integrated contrast of the model is 

n _ By - Bfj 

B H ' 

Combining equations (5) and (6): 

C = (K —1)4^. 


( 5 ) 


( 6 ) 

(7) 


Model Studies 

A typical polar curve of integrated contrast, of 
the model airplane shown in Figure 34, is shown in 
Figure 35. Similar curves for model aircraft and 
model submarines will be found in OSRD Report 
No. 6553. 43 


CONFIDENTIAL 










134 


THE VISIBILITY OF NAVAL TARGETS 


Average Reflectance 

The integrating contrast photometer can also be 
used to determine the reflectance required if a uni¬ 
form surface perpendicular to the line of sight at 
the model is to have a contrast against the sky equal 


210° 180° ISO 0 



Figure 35. Typical polar curve of integrated inherent 
contrast for the model airplane shown in Figure 34. 

to the integrated contrast of the target. This has 
been called the average reflectance o of the model. 
It is related to the average brightness of the model 
by the relation 

Bm — 9 E, ( 8 ) 

where E is the illumination on a plane perpendicular 
to the line of sight at the model. 

E can be measured in the following manner. Let 
the model be replaced by any uniform flat gray 
surface of reflectance p. This surface should be 
large enough to fill the field of view of photometer 
B and should be mounted perpendicular to the line 
of sight. Let the ratio of the light entering photom¬ 
eter B to the light entering photometer A be desig¬ 
nated by K'. Then 

K ' = qE/B h . (9) 

Equations (7), d8), and (9) can be combined to 
show that the average reflectance of a model is re¬ 
lated to its integrated contrast by the relation 

e = -jL(C + i). not 


4.10.3 ^ Contrast Photometer for Field Use 

A simple, rugged contrast photometer for field 
use was constructed by the Eastman Kodak Com¬ 
pany under Contract OEMsr-1070 (Figure 36). The 
instrument consists of a gray plastic case pierced 
by a central hole through which the background can 
be viewed. A circular, transparent, absorbing wedge, 
contained within the case, covers this hole. In use, 



Figure 36. A contrast photometer for field use. 

With this instrument, the integrated contrast of any target 
can be determined if the average reflectance of the target is 
known. 

the photometer is held with its surface perpendicu¬ 
lar to the line of sight as shown in Figure 37. The 
wedge is then rotated by the thumbs of the operator 
until the hole appears to be as bright as the surface 
of the case. The integrated contrast of any target 
of average reflectance p is related to the scale read¬ 
ing of the photometer {H) by the relation 

( 11 ) 

A small nomographic chart representing equa¬ 
tion (11) may be mounted on the back of the pho¬ 
tometer to facilitate its use in the field. One of these 
instruments was turned over to the Navy for use 
in the Pacific. 


CONFIDENTIAL 

















A CONTEMPLATED HANDBOOK OF VISIBILITY 


135 



Figure 37. Contrast photometer in use. 

Glare from the sky is eliminated by means of special goggles 
which limit the operator’s field of view to the surface of the 
photometer. 

410,4 The Sun-Ratio 

The integrated contrast of a ship or a plane de¬ 
pends on the nature of the lighting conditions, for 
these determine the pattern of highlights and shad¬ 
ows. The most widely used index of the lighting 
conditions is called the sun-ratio. It is, by definition, 
the ratio of the illumination on a vertical surface 
facing the sun to the illumination on a vertical sur¬ 
face facing away from the sun. The sun-ratio varies 
from unity on a uniformly overcast day to 30 or 
more near sunrise or sunset in clear weather. 

Figure 38 shows one of several types of sun-ratio 
meters which were built or tested. 8c An earlier model 
of this instrument was used by the Tiffany Founda¬ 
tion in conjunction with the integrating contrast 
photometer on High Hill, the shapes of the polar 
curves depending upon the value of sun-ratio. 

411 A CONTEMPLATED HANDBOOK 
OF VISIBILITY 

At the outset of its research on the visibility of 
targets, Section 16.3 was asked by the Navy to pre¬ 
pare a handbook of visibility suitable for use under 
operational conditions by nonspecialized personnel. 
Considerable thought was given to the preparation 


Figure 38. Sun-ratio meter. 

A double-sided mirror divides the integrating sphere, so that 
the two values of illumination can be measured simultaneously 
by the two barrier-layer type photoelectric cells. 

illustrated by sketches or photographs and based 
upon practical problems submitted by Navy liaison 
officers. These problems, typical of situations en¬ 
countered under operational conditions, would have 
been implemented by special charts and tables de- 


of such a volume, and a draft was begun by the sec¬ 
tion chief. However, the requisite experimental 
data did not become available in time for the manu¬ 
script to be completed. 

The handbook of visibility, as planned, would 
have been limited to the visibility of naval targets 
along horizontal paths of sight. It would have con¬ 
tained, in concise form, much of the information to 
be found in this chapter, together with nomographic 
visibility charts for circular targets (Figures 2 
through 10). The principal feature, however, was 
intended to be a large number of worked examples, 


CONFIDENTIAL 







loU 7- 0 —p— I 0 0 


136 


THE VISIBILITY OF NAVAL TARGETS 


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CONFIDENTIAL 


Figure 39. The nomograph is used by placing a straightedge across the chart in such a manner that it joins the appropriate points on the three 
vertical lines. (See text.) 







VISIBILITY UNDER OPERATIONAL CONDITIONS 


137 


signed to facilitate the calculations. Figure 39 shows 
an example of one of the charts prepared for the 
handbook. This nomograph indicates, for any height 
of the observer above the sea, the height of target 
which will be seen in line with the horizon. This 
chart involves no trigonometric approximation, and, 
therefore, values obtained with it take precedence 
over those obtained by means of Bowditch’s rule. 44 



-20 -16 -12 -8 -4 0 4 8 12 16 20 

SOLAR DEPRESSION IN DEGREES 

Figure 40. Plot of values of sky brightness as a func¬ 
tion of solar depression compiled from data published 
by the staff of the U. S. Weather Bureau. Moderate 
overcast may be allowed for by lowering the value of 
sky brightness by a factor of 10. 

For targets more distant than the horizon, use 
the right-hand (outside) scale marked Height of 
Target and the right-hand scale marked Target Dis¬ 
tance. In the case of targets which are between the 
observer and the horizon, use the left-hand (inside) 
scale marked Height of Target and the left-hand 
scale marked Target Distance. 


Another example of the charts planned for the 
handbook of visibility is shown in Figure 40. Com¬ 
piled from data published by the staff of the U. S. 
Weather Bureau, 52 ’ 53 this figure indicates average 
values of sky brightness as a function of solar alti¬ 
tude. It is intended to serve as a guide in selecting 
the appropriate value of adaptation brightness 
( B h ) during sunrise or sunset, when the brightness 
of the sky undergoes a millionfold change of bright¬ 
ness within a few minutes. The value of solar alti¬ 
tude at any time, date, and location can be com¬ 
puted from standard navigation tables. 

412 VISIBILITY UNDER OPERATIONAL 
CONDITIONS 

In predicting the visibility of naval targets from 
the visibility charts contained in this chapter, it 
should be borne in mind that the data represent the 
performance of excellent observers under nearly 
ideal observing conditions. Because of fatigue, dis¬ 
comfort, distraction, and the necessity for search, 
it is to be expected that, in most instances, actual 
sightings at sea will occur at ranges somewhat less 
than those indicated by the charts. On the other 
hand, the atmosphere is sometimes so inhomoge¬ 
neous that the actual sighting range may exceed the 
range indicated by the charts. Experience in the 
use of the nomographs is the best guide to the allow¬ 
ances that should be made for departures from the 
conditions upon which the charts are based. Under 
no circumstances should the visibility charts con¬ 
tained in this chapter be used to predict the ability 
of aviators to see objects on the ground. This prob¬ 
lem is dealt with in the following chapter. 


CONFIDENTIAL 




















Chapter 5 

VISIBILITY FROM AIRCRAFT 


si INTRODUCTION 

T he minimum contrast required to make an object 
on the ground visible from the air can be pre¬ 
dicted by methods similar to those presented in the 
preceding chapter. However, along any slant path, 
the changes of atmospheric conditions with altitude 
must be taken into account. This may be accom¬ 
plished by means of the nomographic visibility 
charts presented in this chapter. 

5.2 STRATIFICATION OF THE 

ATMOSPHERE 

Airmen view the earth along slant paths through¬ 
out which the scattering and absorbing particles 
vary in number and kind. The idealized case of a 
homogeneous atmosphere exhibiting regular, con¬ 
tinuous stratification was discussed in Chapter 2, 
wherein it was shown that the law of contrast atten¬ 
uation along slant paths in such an optical standard 
atmosphere could be expressed in a simple form 
[equation (36), Chapter 2] in terms of the optical 
slant range R. This distance is related to the actual 
slant range R by equation (29) of Chapter 2. Fig¬ 
ures 1, 2, and 3 are plots of this equation for various 
values of (9, the angle between the line of sight and 
the horizontal. Values of true altitude are indicated 
by the family of dashed curves. These figures have 
been called optical slant-range diagrams. 

5-21 Discontinuous Stratification 

Ground Haze 

The curves in Figures 1, 2, and 3 apply only when 
no optically dissimilar strata are present. Ordi¬ 
narily, however, the air near the ground contains 
dust, smoke, and large water particles not found at 
higher altitudes. This condition is frequently called 
ground haze. In clear weather, this layer often has 
a sharply defined upper boundary, above which the 
atmosphere contains very little condensed water. 
Above the boundary, the meteorological range is 
often several times as great as within the ground 
haze. 


Graphical Representation. Figure 4 illustrates 
how the discontinuity in meteorological range can 
be represented on Figure 3. For simplicity, Figure 4 
shows only the curve for 0 — 25 degrees in Figure 
3. Let it be assumed that the upper boundary of the 
ground haze is at an altitude of 5,000 feet and that 
the meteorological range is five times greater above 
the boundary than below it. Beginning at the point 
corresponding to an altitude of 5,000 feet, a new 
curve has been drawn having five times the_slope of 
the original curve. The relation between R and R 
is then represented by the accentuated curve; it 
follows the normal curve up to altitude 5,000 feet 
and the steeper curve thereafter. 

Diffuse Boundaries. If the boundary of the ground 
haze is gradual rather than sharp, the accentuated 
curve in Figure 4 may be rounded off to avoid the 
abrupt change in slope. 

The character and altitude of the boundary can 
be observed easily from a plane climbing or de¬ 
scending through it. In many cases the pilot can 
also make an estimate of the ratio of the meteoro¬ 
logical range above and below the boundary. Pro¬ 
ficiency in describing the stratification of the atmos¬ 
phere is acquired very quickly by any flyer, once 
he understands what to look for. Moreover, stratifi¬ 
cation may be correlated with other meteorological 
conditions, and experience may enable very intelli¬ 
gent guesses to be made by an observer on the 
ground. Statistical information concerning the fre¬ 
quency of occurrence of common stratification con¬ 
ditions in a given locality can be accumulated in 
the same manner as other meteorological data. 

Ceilings 

Experience in drawing curves of modified slope 
on the optical slant-range diagram is quickly ac¬ 
quired with practice. Freehand curves are usually 
as precise as are warranted by the estimates of the 
ratio of meteorological ranges within the strata. 
Often, straight lines are sufficient approximations 
for curves of very great or very small slopes. An 
example of the latter is the ceiling. This is an opaque 
cloud deck within which the meteorological range 
is very short. Usually, the lower boundary of the 
cloud layer is sharply defined; it can be represented 


138 


CONFIDENTIAL 


SLANT RANGE (YARDS) 


STRATIFICATION OF THE ATMOSPHERE 


139 


OPTICAL SLANT RANGE (YARDS) 
[VALUES OF r] 



OPTICAL SLANT RANGE (YARDS) 

[values OF r] 

Figure 1. Optical slant-range diagram for the optical standard atmosphere. 

Solid curves represent the relation between R and R for various sight path elevation angles 0. Broken lines represent loci of equal 
altitude, expressed in feet. 


CONFIDENTIAL 


SLANT RANGE (FEET) 

































































































































































































































































































































































































































SLANT RANGE (YARDS) 


140 


VISIBILITY FROM AIRCRAFT 


OPTICAL SLANT RANGE (YARDS) 
[VALUES OF rJ 



[values OF r] 


6 5,0 0 0 


6 0,0 0 0 


55,000 


5 0,00 0 


4 5,000 


4 0,0 00 


35,000 


30.000 


2 5,0 00 


2 0.0 00 


l 5,000 


I 0.0 0 0 


5.0 0 0 


0 


Figure 2. Optical slant-range diagram similar to Figure 1, but adapted to the solution of problems involving shorter 
slant ranges. 


CONFIDENTIAL 


SLANT RANGE (FEET) 





























































































































































































SLANT RANGE (YARDS) 


STRATIFICATION OF THE ATMOSPHERE 


141 


OPTICAL SLANT RANGE (YARDS) 
[VALUES OF r] 



OPTICAL SLANT RANGE (YARDS) 

[values of r] 


Figure 3. Optical slant-range diagram similar to Figures 1 and 2, but adapted to the solution of problems involving 
still shorter slant ranges. 


CONFIDENTIAL 


SLANT RANGE (FEET) 



























































SLANT RANGE (YARDS) 


142 


VISIBILITY FROM AIRCRAFT 


OPTICAL SLANT RANGE (YARDS) 
[VALUES OF 



OPTICAL SLANT RANGE (YARDS) 

[values of r] 


Figure 4 . Optical slant-range diagram for 6 — 25 degrees. (Not intended for us in solving problems.) 

Accentuated curve shows relation between R and R when the ground haze has a sharp upper boundary at 5,000 feet, above which 
the meteorological range is five times greater than it is below the boundary. 


CONFIDENTIAL 


SLANT RANGE (FEET) 


































































SLANT RANGE (YARDS) 


STRATIFICATION OF THE ATMOSPHERE 


143 


OPTICAL SLANT RANGE (YARDS) 
[VALUES OF r] 



OPTICAL SLANT RANGE (YARDS) 
[VALUES OF r] 


Figure 5. Optical slant-range diagram similar to Figure 4 except for a ceiling at 10,000 feet. (Not intended for use 
in solving problems.) 


CONFIDENTIAL 


SLANT RANGE (FEET) 













































144 


VISIBILITY FROM AIRCRAFT 


on the optical slant-range diagram by a horizontal 
straight line passing through the point on the stand¬ 
ard curve which corresponds to the altitude of the 
ceiling (Figure 5). 

Ceiling Zero. Even when the ceiling is at ground 
level the nomographic visibility charts contained in 
this chapter can be used to predict the visibility of 
objects along slant paths. However, it should be 
noted that the relation between R and R within the 
haze blanket is given by the printed curves on the 
optical slant-range diagram rather than by a hori¬ 
zontal line. The slope of the curves on the optical 
slant-range diagram does not depend upon the mag¬ 
nitude of the meteorological range. The slope of 
hand-drawn sections is governed by the ratio of the 
meteorological ranges above and below the stratum 
boundary. When fog extends to the ground, the 
limitation it imposes on liminal target distance is 
taken into account by the value of meteorological 
range entered on the nomographic visibility charts, 
the relation between R and R being expressed by 
the printed curves on the optical slant-range dia¬ 
gram. 

5.3 NOMOGRAPHIC METHODS 

The concept of optical slant range enables nomo¬ 
graphic visibility charts of the type discussed in 
Section 4.6.1 to be used to predict the visibility of 
objects along slant paths. The nomographic chart 
shown in Figure 19 is constructed around contrast 
reduction equation (3), Chapter 4. This equation is 
of the same form as equation (36), Chapter 2, which 
expresses the law of contrast attenuation along 
slant paths in terms of the optical slant range. Fig¬ 
ure 19 can be adapted for use in predicting visibility 
from aircraft by changing the legend of the scale 
marked “Liminal Target Distance” to read “Values 
of R” 

5,3,1 Visibility Charts for Aerial Use 

A series of nomographic visibility charts for cir¬ 
cular and rectangular targets at decimal levels of 
adaptation brightness are presented in Figures 6 
through 30. Each of the twenty-five figures (Figures 
6 through 30) is a nomographic visibility chart for 
uniform circular or rectangular targets seen by an 
observer whose eyes are adapted to the value of 
brightness indicated at the lower right corner of the 
diagram. The shape of target to which a chart ap¬ 


plies is indicated at the lower left corner of the dia¬ 
gram. When used in the manner described in follow¬ 
ing text, these charts enable the visibility of targets 
on the ground to be predicted. 

The assumed value of adaptation brightness 
( B h ) is indicated at the lower right corner of 
each chart. The descriptive phrases, such as Full 
Daylight or Quarter Moon, are intended to serve 
as a .rough guide in selecting the proper chart 
for solving a particular problem. In making the 
selection, however, it should be borne in mind that 
the level of brightness to which an aerial observer’s 
eyes are adapted depends upon the average reflect¬ 
ance of the terrain at which he is looking. Therefore, 
the descriptive phrases are applicable only to cir¬ 
cumstances when the sky-ground ratio is approxi¬ 
mately unity, or when there is sufficient haze to 
make the apparent brightness of the earth approach 
the equilibrium value (Section 2.3.4). Otherwise, a 
chart for a lower or higher value of B H should be 
used. For example, on a very clear but overcast day, 
Figure 7 should be used to predict the visibility of 
circular objects on a large field of snow, but Figure 
8 should be used to predict the visibility of such 
objects in a verdant landscape for which the sky- 
ground ratio is 10. 

Projected Target Area 

Before the visibility of an object on the surface 
of the earth can be predicted, its projected area 
must be determined. For example, the projected area 
of a flat, level surface is simply its true area multi¬ 
plied by the sine of 6. In the case of targets that are 
not flat, level surfaces, the projected area can be 
determined graphically by techniques known to 
every draftsman. The projected areas of existing 
structures can be determined from properly made 
oblique aerial photographs. 

Effect of the Atmosphere. Because the atmosphere 
along the line of sight is stratified, the slant range 
ordinarily exceeds the optical slant range. There¬ 
fore, the target actually subtends a smaller angle at 
the observer’s eye than if it were at distance R. 
From the standpoint of the user of the nomographic 
visibility charts, the atmosphere is equivalent to an 
optical system producing demagnification. 

It was suggested in Section 4.8 that the magnify¬ 
ing effect of binoculars can be allowed for by enter¬ 
ing an increased value of target area into the nomo¬ 
graphic visibility charts. Similarly the “demagnify- 
ing” effect of the atmosphere along slant paths can 


CONFIDENTIAL 



VALUES OF R (YARDS) 


NOMOGRAPHIC METHODS 


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VALUES OF R (YARDS) 


NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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VALUES OF R (YARDS) 


NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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VALUES OF R (YARDS) 


NOMOGRAPHIC METHODS 


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VALUES of r (yards) 


NOMOGRAPHIC METHODS 


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VALUES OF R (YARDS) 


NOMOGRAPHIC METHODS 


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NOMOGRAPHIC METHODS 


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VALUES OF R (YARDS) 


NOMOGRAPHIC METHODS 


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194 


VISIBILITY FROM AIRCRAFT 


be allowed for by entering the visibility charts with 
a reduced value of target area. 

The reduced or effective projected target area A 
is related to the projected target area A by the ex¬ 
pression: 

MSP 

A nomographic chart embodying this relation is 
shown in Figure 31. 

54 THE REFLECTANCE OF NATURAL 
TERRAINS 

Natural terrains form the background for most 
objects seen from aloft. Protective concealment has 
been attained when, to enemy eyes, the object is in¬ 
distinguishable from the surrounding terrain. The 
design of camouflage by engineering methods must, 
therefore, be based upon knowledge of those optical 
properties of natural terrains which determine the 
appearance of the earth from aloft. The experiments 
described in this section provide some of the re¬ 
quired information. 

5 41 The Spectrogeograph 

A specially constructed spectrograph adapted for 
aerial use was required for the study of the optical 
properties of natural terrains. Under the provisions 
of contract OEMsr-717, the Eastman Kodak Com¬ 
pany undertook “the design and construction of an 
instrument and the development of techniques for 
its use in measuring the quantity and spectral qual¬ 
ity of radiant energy from natural daytime sources 
reaching an aeroplane during flight.” The instru¬ 
ment (see Figure 4, Chapter 1) was called a spectro¬ 
geograph at the outset of the program, when it was 
thought that it would embody the principle of the 
well-known spectroheliograph. The name was re¬ 
tained as a code word for use in unclassified corre¬ 
spondence after the spectroheliograph principle had 
been abandoned. 11 

The spectrogeograph is essentially a photographic 
spectroradiometer capable of measuring the spectral 
distribution of radiant energy reaching an airplane 
from the ground or from the sky. The spectral re¬ 
flectance of natural terrains can be determined by 
comparison of the energy received from any terrain 
with that received from gray panels of several 


known reflectances located in or near the terrain. In 
this limited sense, the spectrogeograph may be con¬ 
sidered to be an aerial spectrophotometer. 

Prior Art 

Two previous investigations contributed to the 
design of the spectrogeograph: 

1. A four-lens aerial camera in which various 
filters were tested for their effect in reducing haze 
in aerial photographs is described in a monograph 
on the theory of photography, Number 4, from the 
Research Laboratories of the Eastman Kodak Com¬ 
pany. The title of this monograph is Aerial Haze 
and Its Effect on Photography from the Air. 55 

Photographs were taken from the air with this 
camera at Rochester, New York, and at Langley 
Field, Virginia, in 1918 and 1919. The method con¬ 
sisted essentially in photographing three test objects 
(a black, a gray, and a white canvas, each 60x60 
feet, of known reflectance) spread upon level ground. 
Four lenses, each of 10-inch focal length, were lo¬ 
cated in a single lens board. The plate holder carried 
four 4x5-inch plates. Provision was made in each 
lens barrel for the insertion of color filters. The 
camera thus served as an abridged photographic 
spectrophotometer, since the filters chosen were 
such as to divide the spectrum into sharp intervals 
of known limits. The reduction of contrast between 
the images of the targets, when photographed on 
various days, at various altitudes and with various 
degrees of haze, was determined by photographic 
photometry. 

2. An aerial spectrograph was employed for a 
similar purpose by R. Schimpf and C. Aschenbren- 
ner in 1934 and 1939. Their investigation was de¬ 
scribed in the Zeitschrift fur angewandte Photo¬ 
graphies Vol. II, pp. 41-51 (1940). The spectro¬ 
graph consisted of a direct-vision Amici prism be¬ 
tween collimator and objective lenses. A collector 
lens was placed directly below the spectrograph 
slit and an image of the ground was focused on the 
prism (Figure 32). A step wedge was placed in con¬ 
tact with the slit so that the spectrum was divided 
into seven intensity bands for calibration purposes. 
The exposure time was standardized at 5 seconds, 
with the result that the image of a long strip of ter¬ 
rain swept across the prism during the exposure. 
The spectrogram obtained with this arrangement 
is, of course, that of the average illumination from 
a very large area. Neutral-density filters were used 
in front of the collector lens to reduce all exposures 


CONFIDENTIAL 



THE REFLECTANCE OF NATURAL TERRAINS 


195 


I 0,00 0,000^ 
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equation (1). 


i n 
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2 0 0 


2 0,0 0 0 - 


4 0,0 0 0- 

6 0,0 0 0 - 

8 0,0 0 0 - 
00 , 000 - 


2 0 0,0 0 0 - 

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6 0 0,0 0 0 - 


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1,0 0 0,0 0 0-^^ 


==■ 1,000,000 

1 - 2,0 00,00 0 

j- 4.0 00,0 0 0 
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- 8.0 0 0.0 0 0 
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the effective projected area of a target. This nomograph represents 


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= 800 
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CONFIDENTIAL 


PROJECTED TARGET AREA SQ.FT. 






































































196 


VISIBILITY FROM AIRCRAFT 


to a pre-established level. The aerial spectrograph 
was mounted directly on the floor boards of the air¬ 
plane, so that in normal straight flight the exposure 
was vertically downward. 

Three terrains of typical nature and sufficient ex¬ 
panse and uniformity were selected in the vicinity 
of Berlin, namely, a meadow, a forest, and a lake. 
Immediately prior to each flight, a comparison- 
reflecting surface lying horizontally on the runway 



Figure 32. Schimpf and Aschenbrenner aerial spec¬ 
trograph. 


was photographed from a height of 2.5 meters. The 
several terrains were then photographed from alti¬ 
tudes of 100, 1,000, and 2,000 meters. The spectral 
reflectances of the terrains were then deduced from 
the densities of the spectrograms of the comparison 
surface and the terrains. 

Strip-Camera Principle 

The spectrogeograph had its inception when an 
instrument modeled on the principle of the spectro- 
heliograph was conceived. In the first conception 
(see Figure 33), an objective lens was to form an 
image of the ground on the slit of a spectrograph. 
At a selected wavelength in the focal plane of the 
spectrograph, a second slit was to be located in front 
of a moving strip of film. The rate of movement of 
this film was to be synchronized with the rate of 
movement of the image of the ground, so that a con¬ 
tinuous strip photograph of the ground would be 
produced by light of substantially a single wave¬ 
length. The location of the slit in the focal plane 
was to be adjustable. With an instrument of this 
kind, each wavelength region would require a sepa¬ 
rate flight over the target. Since daylight changes 
continually, there could be no assurance that the 
results for the several wavelengths, recorded dur¬ 
ing successive passages over the terrain, were com¬ 
parable. 

Such an instrument might be useful for studies 
in which only one wavelength region need be con- 



Figure 33. Moving-film type of spectrograph. 


CONFIDENTIAL 





















THE REFLECTANCE OF NATURAL TERRAINS 


197 


sidered, particularly in attempts to “break” camou¬ 
flage, but experience with aerial strip cameras does 
not encourage the use of this method for photo¬ 
graphic photometry, which imposes the most severe 
requirements of uniformity and reproducibility of 
exposure. Consequently, an alternative to the strip- 
camera principle was sought. 

Image Stabilization by Optical Means 

Refraction in a thick, rotating glass plate has 
been used in high-speed motion picture photography 
by synchronizing, during a sufficient exposure time, 
the motion of the refracted image with film moving 
continuously at a high speed. The converse of this 
principle was finally applied in the spectrogeograph. 
The refraction in the glass block produces a dis¬ 
placement equal and opposite to the rate of motion 
of the image of the ground in the focal plane of an 
aerial camera objective. In this way the image is 
held stationary upon the slit of the spectrogeograph 
during an exposure time three or four times as great 
as would be obtained in a strip camera with the 
same slit width. The film is stationary during the 
exposure, no slit is used in the focal plane of the 
spectrograph, and a complete spectrogram of a strip 
of the image is obtained in a single exposure. 

The two principal problems in connection with 
this arrangement are the synchronization of the 
block with the movement of the image formed by 
the objective lens and the identification of the strip 
actually analyzed in the spectrogram. Even if a 
spectrogram were taken during each quarter revolu¬ 
tion of the stabilizer block, transverse strips of the 
ground would be missed, the width of which would 
be at least twice the width of the strips analyzed. 
It is, therefore, necessary to aim as well as to syn¬ 
chronize the spectrogeograph. 

Target Identification 

An identification camera is necessary to identify 
accurately the portion of the terrain analyzed by 
the spectrograph. Use of a separate identification 
camera was considered when the stationary-film 
type of spectrogeograph was being designed. The 
identification camera could be synchronized electri¬ 
cally with the opening of the spectrograph slit. Such 
an arrangement would have considerably simplified 
the construction and operation of the spectrogeo¬ 
graph. The idea was rejected, however, because of 
alleged installation limitations and because vibra¬ 
tion of the plane might disturb the parallelism of 


alignment of the separate camera and spectrograph 
and introduce errors into the identification of the 
portion of the image analyzed. 

In the first attempt to design a built-in identifica¬ 
tion camera, a mirror was placed in the spectro- 
geograph to reflect the light onto the identification 
camera. This mirror was to be hinged and the plan 
was to rotate it automatically out of the axis of the 
spectrogeograph, preliminary to the spectrum analy¬ 
sis. Because of the mechanical difficulties of moving 
this large mirror rapidly without causing vibrations 
which would disturb the spectrogeograph during the 
exposure, other types of identification were inves¬ 
tigated. 

A stationary mirror was finally used in the spec- 
trogeograph. This arrangement utilizes the fact that 
the airplane is moving at a relatively constant speed 
toward the selected target. Although the identifica¬ 
tion camera and the spectrograph slit occupy dif¬ 
ferent portions of the focal plane of the objective, 
a photograph of the target which is subsequently 
analyzed by the spectrograph is obtained by syn¬ 
chronizing the time lapse between the identification 
and spectrum exposures with the time of transit of 
the crosswire in the sight. 

The spectrogeograph automatically takes two 
identification pictures for each spectrogram. One 
of these pictures is taken at a time when the se¬ 
lected target is certainly imaged in the identification 
camera, and the other is taken simultaneously with 
the opening of the slit of the spectrograph. These, 
identification pictures overlap and can be assembled 
as a composite picture. It is a simple matter of geo¬ 
metric construction to identify exactly the target 
analyzed, which appears in the composite picture 
at a position known in relation to crossmarks in the 
second identification picture (Figure 34). The spec¬ 
trogram of the target indicated in Figure 34 is 
shown in Figure 35. 

Description of the Instrument 

The spectrogeograph consists of a 24-inch, //6 
aerial objective lens, an optical device which stabi¬ 
lizes a portion of the image on the slit of a grating 
spectrograph, an identification camera, and a sight 
mechanism. The image stabilizer is driven by an 
adjustable governor-controlled motor (Figure 36), 
and it is coupled to the sight with which the syn¬ 
chronization of the image and stabilizer speeds is 
verified and the desired target selected. Electric cir¬ 
cuits, which are controlled chiefly by the sight mech- 


CONFIDENTJAL 



198 


VISIBILITY FROM AIRCRAFT 



h-4.75"- 

Figure 34. Composite identification picture. 



Figure 35. Spectrogram corresponding to Figure 34. 


anism, actuate the shutters of the identification 
and spectrum cameras and initiate the transport of 
the film after each exposure. 

Optical System. The optical system is shown in 
Figure 37. The 24-inch objective lens is provided 


with an iris diaphragm, for control of exposure. A 
graduated knob controlling the aperture is located 
on the right side of the casting below the sight (Fig¬ 
ure 4, Chapter 1). 

The optical stabilizer device, which holds the 
image of the ground stationary upon the spectro- 
graphic slit during the exposure, consists of a glass 
block having polished plane parallel surfaces (Fig¬ 
ure 38). This block is mounted so as to rotate in 
front of the spectrograph slit. The rotation is about 
an axis parallel to the length of the slit. When this 
block is rotated at the proper speed, the refractive 
displacements of rays passing through it compensate 
for the motion of the image of the ground in the 
focal plane of the 24-inch aerial objective lens. The 
provisions for synchronizing the speed of rotation 
of this block with the velocity of the image of the 
ground formed by the objective will be described 
subsequently. 

The Spectrograph. The spectrograph proper (Fig¬ 
ure 39), consists of a slit 3 inches long, collimator 
and camera lenses each of 15-inch focal length and 
3-inch diameter, and a Wood, first-order replica 


CONFIDENTIAL 









THE REFLECTANCE OF NATURAL TERRAINS 


plane grating having 7,500 lines per inch over a 
4-inch square area. 

The film is held in the focal plane of the spec¬ 
trograph in the film magazine of a K-24 automatic 



Figure 36. Photograph of left side of spectrogeograph, 
showing governor-controlled motor, identification and 
spectrum cameras, sky periscope, sky and practice con¬ 
trol lever, coincidence control lever, and periscope 
shutter lever. 

aerial camera, the mechanism of which provides 
high-speed automatic film changes. The focal-plane 
shutter curtain belonging to this mechanism has 
been removed, and the spectrograph exposure is con- 



Figure 37. Sketch of basic optical system of spectro 
geograph. 


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200 


VISIBILITY FROM AIRCRAFT 


trolled by a vane just above the slit (see Figure 
40). 

This vane is opened by a solenoid when the image 
is first synchronized on the slit, remains open as 
long as the image is held stationary on the slit, and 
closes again before the block rotates beyond the 
extreme position for synchronization. The block is 
mounted in a metal cylinder with apertures cut 



Figure 38. Cross sections of stabilizer block and 
cylindrical apertures in relation to slit for successive 
positions of the block. 


away opposite each of the faces (Figure 41). The 
remaining portions of the cylinder prevent light 
from entering or leaving the block at angles in ex¬ 
cess of those for which accurate stabilization is ob¬ 
tained. The slit-vane opens and closes while two 
successive opaque portions of this cylinder are in 
front of the slit. Consequently, the edges of the 
apertures of the cylinder actually control the ex¬ 
posure; the slit-vane merely confines the exposure 
to one of these apertures. 

Identification Photographs. Identification photo¬ 
graphs of the ground are made by reflection of a 
portion of the image formed by the objective lens 
into a second K-24 automatic camera. The exposure 
of the film in this camera is controlled by the focal- 
plane shutter which is a standard part of the mecha¬ 
nism. The film is held accurately in the focal plane 
by being pressed against a clear glass plate through 
which the light must pass. Opaque crosses engraved 
on this pressure plate produce the identification 
marks in the pictures. A Wratten No. 12 haze filter 
is cemented onto the front surface of the pressure 
plate of this camera. 

The identification camera photographs a portion 
of the ground ahead of the section imaged on the 
spectrograph slit. The central ray recorded in the 


identification picture is actually 11 degrees forward 
of the vertical. Consequently, the identification 
camera must photograph the target imaged on the 
spectrographie slit prior to the exposure in the spec¬ 
trograph. The necessary interval depends upon the 



Figure 39. Photograph of interior of spectrogeograph. 
speed of the image in the focal plane and is con¬ 
trolled by the same synchronization device that 
controls the speed of rotation of the stabilizer block. 
This synchronization apparatus is part of the sight 
mechanism. 

The Sight. The sight (Figure 42) is built as an 


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THE REFLECTANCE OF NATURAL TERRAINS 


201 


integral part of the spectrogeograph. A crosswire is 
carried across the focal plane of the sight in a frame 
which travels at a speed proportional to the rate of 
rotation of the stabilizer block. When the crosswire 
remains coincident with the image of a target, that 




Figure 41. Perspective view of stabilizer block, cylin¬ 
der, and coupling between block and sight mechanism. 


target is focused upon the spectrograph slit and its 
image remains stationary while one face of the sta¬ 
bilizer block rotates past the slit. The speed of a 
governor-controlled motor, which drives the sta¬ 
bilizer block as well as the crosswire, is adjusted by 


prior trial so that, at any altitude and ground speed, 
the sight-wire remains fixed on the image of the 
ground. The motor rotates the block continually, but 
the crosswire in the sight moves only when a clutch 
is engaged. 

When the target selected for spectral analysis ap¬ 
pears in the sight and coincides with the crosswire 
in its initial position, the operator presses a handle. 



This engages the clutch, and the crosswire moves, 
remaining coincident with the image of the target. 
When the crosswire reaches a position correspond 
ing to the first position of the stabilizer block in 
which the target is imaged on the slit, an electric 
contact on the frame which carries the crosswire 
completes a circuit through a solenoid which opens 
the vane above the slit, thus exposing the spectro¬ 
gram. The speed of the crosswire can be varied by 


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202 


VISIBILITY FROM AIRCRAFT 


means of the governor control and the gear shift 
over a 16:1 speed ratio. This range of speeds en¬ 
ables the image of the ground to be stabilized at 
all altitudes between 1,000 and 16,000 feet, pro¬ 
vided the spectrogeograph is flown at a ground speed 
of 150 miles per hour. The image may be stabilized 
at higher altitudes if the ground speed of the air¬ 
plane is increased. 

The width of the spectrograph slit is changed 
from 0.05 inch to 0.15 inch when the gear ratio is 
changed from low to high. This is accomplished with 
a lead screw which is coupled to the gear-shift lever 
(Figure 43). This arrangement partially compen¬ 
sates for the change of exposure time, which is 
directly proportional to the time of rotation of the 
stabilizer block. The use of the narrow slit for high 
altitudes also makes possible the analysis of a 
smaller area. The change of spectral resolution cor¬ 
responding to this change of slit width is not impor¬ 
tant in the analysis of the spectrally continuous 
energy distributions which are observed from air¬ 
craft. 


Periscope and Sky Mirror System. A mirror has 
already been mentioned which may be turned to a 
position so as to reflect light from the top of the 
airplane into the spectrograph slit. Light from the 
sky is brought to this mirror by a periscope system 
(Figure 44), which is designed specifically for in¬ 
stallation in a B-17 Flying Fortress. 

A flat, opal-glass cap is provided for the top of 
the periscope tube. The light transmitted by the 
periscope from this opal glass is representative of 
the illumination on a horizontal plane. Compensa¬ 
tion for the selective absorption of this opal glass 
and of the other optical elements of the periscope 
must be accomplished by the calibration proce¬ 
dures. 

A pair of adjustable mirrors is also mounted in an 
accessory which may be placed on top of the peri¬ 
scope (Figure 45 ). The light from any region of the 
sky can be analyzed by this mirror system by ori¬ 
enting it with respect to the direction of flight of the 
airplane and by adjusting the mirrors to reflect light 
from various vertical angles into the periscope. 



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THE REFLECTANCE OF NATURAL TERRAINS 


203 


\ 



Figure 44. Perspective sketch of periscope system. 


Oblique Terrestrial Mirror System. A large ad¬ 
justable mirror system is provided for the investi¬ 
gation of the spectral distribution of energy re¬ 
ceived from the ground at oblique angles (Figure 
46). This system can be placed in front of the 
objective lens of the spectrogeograph. With this 
mirror system, the spectral distribution of the en¬ 
ergy from targets as far as 65 degrees from the 
vertical can be analyzed. These oblique angles are 
perpendicular to the direction of the flight of the 
airplane, so that flight paths can be laid out and 
specific target areas investigated. 

Installation of the Spectrogeograph 
The spectrogeograph was designed to be flown in 
a B-17 aircraft partly because of advice from liaison 
officers that this type of airplane was more likely 
to be available than any other type large enough 
to contain the instrument and partly because the 
B-17 has a removable hatch in the top of the fuse¬ 


lage directly over the camera station. This permits 
the periscope of the spectrogeograph to protrude 
above the airplane when light received from above 
is to be measured. 

Figure 47 shows the spectrogeograph installed in 
the camera pit of the B-17F(F-9) airplane (Army 
229801) in which it was flown during the experi¬ 
ments in Florida and California. The instrument 
and its mounting frames weigh approximately 250 
pounds. The main frames are welded steel parallelo¬ 
grams which are bolted to the standard camera sup¬ 
ports in the B-17 and clamped to the floor-support 
member at the rear of the camera pit. The spectro¬ 
geograph, in a modified A-8 ring mount, is bolted 
to the tops of the frames. The mounting is so de¬ 
signed that the instrument is offset toward the tail 
of the ship in order to give the sight system a suffi¬ 
cient forward view. This has the additional advan¬ 
tage of transferring at least one-third of the weight 
of the installation to the floor structure. The mount- 


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204 


VISIBILITY FROM AIRCRAFT 



Figure 45. Perspective sketch of sky-scanning mirror 
system. 



Figure 46. Perspective sketch of large mirror system 
for oblique terrestrial analyses. 


ing is relatively free from sidesway, but this effect 
is further eliminated by auxiliary welded steel 
frames, triangular in shape, which are bolted to the 
sides of the main frames and rest on the floor at the 


right and left of the camera pit. These triangular 
frames, clearly shown in Figure 47, serve to transfer 
most of the weight of the spectrogeograph to the 
floor in case of hard landings. 

Calibration and Use 

The spectrogeograph is not an easy instrument 
to use. Technical skill of a high order and meticu¬ 
lous attention to detail are required, not only at 
the time of the flight but later in the laboratory. 



Figure 47. Spectrogeograph installed in camera pit 
(radio room) of a B-17 airplane. 

Periscope protrudes through open hatch in roof directly above 
camera station. (Photograph by Photo-Technical Unit, AAFTAC, 
Orlando, Florida.) 

Useful results can be obtained only if the experi¬ 
ments are planned and the findings interpreted by 
a scientist thoroughly versed in the physical prin¬ 
ciples involved. 

In an effort to preserve the experience gained by 
Section 16.3 of NDRC and its contractors, the Tif¬ 
fany Foundation has prepared a report entitled 
Calibration and Use of the Spectrogeograph, which 
is intended to serve as an instruction manual for 
subsequent users of the instrument, as well as a 
record of the procedure by which the data were ob¬ 
tained. 15 


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THE REFLECTANCE OF NATURAL TERRAINS 


205 


Special Photographic Films 

A special photographic film was required to per¬ 
mit the spectrum to be photographed without serious 
overexposure or underexposure at any wavelength 
between 430 and 900 millimicrons. After many ex¬ 
periments, a double-coated film was chosen. This 
consists of an infrared-sensitive emulsion coated on 
a supersensitive panchromatic film. The spectral 
sensitivity of this material is shown in Figure 48. 
This curve was determined from exposures made in 
the spectrogeograph. The density-versus-exposure 
gradient of this film is very different for the infra¬ 
red than for the visible portion of the spectrum, and 
this fact complicates sensitometry, especially near 
the extreme red end of the panchromatic sensitivity. 

Densitometer 

Measurement of the densities of the numerous 
calibration films and of all of the aerial spectrum 
analyses represents a formidable task. A photo¬ 
electric densitometer is provided to facilitate this 



Figure 48. Spectral sensitivity of double-coated, pan¬ 
chromatic, infrared film. 


work (Figure 49), which is designed to measure the 
densities of very small areas and to locate the film 
very accurately in the measurement beam. Accurate 
location of the film is necessary so that the portion 
of the film corresponding to the selected target can 



Figure 49. Densitometer and power amplifier. 


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206 


VISIBILITY FROM AIRCRAFT 



Figure 50. Mechanical stage and reticle of densitometer. 


be measured with certainty, and so that the wave¬ 
lengths corresponding to the measured densities can 
be assigned positively. The film is placed on a me¬ 
chanical stage which has independent rack and 
pinion movement in vertical and horizontal direc¬ 
tions. This mechanical stage also carries a glass 
pattern which indicates wavelengths and slit posi¬ 
tions by perpendicular lines (Figure 50). When the 
vertical reticle line corresponding to the desired 
wavelength is coincident with an indicator fixed to 
the base of the densitometer, the light beam passes 
through the portion of the spectrogram exposed by 
that wavelength. The stage may then be racked 
vertically until the horizontal reticle line corre¬ 
sponding to the position of the target on the slit is 
also over the fixed indicator. As soon as this process 
is complete, the density of the image of the target 
for the desired wavelength is automatically indi¬ 
cated by the densitometer scale. 

The densitometer consists essentially of a neon 
crater-discharge lamp located in a tube above the 
spectrogram; a 16-mm, 0.20 N.A. microscope ob¬ 
jective located in the bottom of the lamp tube so 
that a reduced image of the discharge crater is 


formed on the spectrogram; a circular w T edge of 
continuously varying optical density located di¬ 
rectly under the spectrogram; a pair of condenser 
lenses which refocus the image of the crater in a 
918 photocell; a second crater-discharge lamp which 
throws a comparison beam into the photocell; an 
amplifier; and a motor which is controlled by the 
output of the amplifier and rotates the optical 
wedge to the balancing position (Figure 51). 

The crater-discharge lamps are excited with 60- 
cycle current and biased with sufficient d-c voltage 
so that they are not extinguished. The flux from 
these lamps fluctuates very nearly sinusoidally; the 
fluctuations of the two lamps are exactly out of 
phase. The photocell is illuminated with a combina¬ 
tion of the light from the two lamps. When the op¬ 
tical wedge is at the balanced position, the peaks 
of the fluctuations of the photocell illumination 
originating in one of the lamps just compensate for 
the troughs of the illumination from the second 
lamp, and the 60-cycle component of the photocell 
current is eliminated. However, if the wedge is not 
exactly at balance, the fluctuations from one of the 
lamps predominate and the resultant 60-cycle fluc- 


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THE REFLECTANCE OF NATURAL TERRAINS 


207 




tuations of the photocell current are amplified. The 
output of the amplifier passes through the two 
shading-coils of a Barber-Colman motor (Figure 
52). The phase of the fluctuations and therefore the 
direction of the rotation of the motor depend upon 
which of the lamps is more strongly illuminating 
the photocell. The connections are arranged so that 
the wedge rotates in the direction which equalizes 
the flux reaching the photocell from the two lamps. 
When the balance is attained, the motor stops for 
lack of 60-cycle current in the shading-coils, and 
the density of the sample is indicated by the loca¬ 
tion of a density scale marked along the rim of the 
wedge. 45 ’ 46 * 47 

The section of the film measured by this den¬ 
sitometer is a circle 0.03 inch in diameter. The pho¬ 
tocell sensitivity extends through the visible and the 
infrared to 1.1 microns, with maxima near 0.45 and 
0.8 microns and with a minimum near 0.5 microns. 
The wavelength centroid of the energy measured by 
the densitometer appears to be at about 0.7 micron, 
although the effective sensitivity corresponds to 
many neon lines scattered over a very wide wave¬ 


length range. The densitometer is calibrated to indi¬ 
cate densities of photographic film equal to those 
indicated by visual densitometers. However, it can¬ 
not be expected to indicate visual densities for ma¬ 
terials which are appreciably different from gray 
photographic silver deposits in either spectral or 
diffusing characteristics. Some photographic devel¬ 
opers produce stained images of a character which 
would be measured incorrectly, but use of the de¬ 
veloper and procedure recommended in this report 15 
will produce deposits which can be measured accu¬ 
rately with the densitometer described. 

The densitometer was shipped throughout the 
country by air and railroad and functioned satisfac¬ 
torily and consistently with only minor adjustments 
and replacements. 

5,4,2 Data from the Spectrogeograph 

Two kinds of information obtainable with the 
spectrogeograph are useful in the design of camou¬ 
flage by engineering methods, data concerning the 
optical properties of the atmosphere along slant 


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208 


VISIBILITY FROM AIRCRAFT 



CONFIDENTIAL 
















































































































































THE REFLECTANCE OF NATURAL TERRAINS 


209 


paths and data concerning the reflectance of natural 
terrains. 

Atmospheric Data 

Repeated attempts were made to obtain observa¬ 
tions suitable for testing the theory and assumptions 
underlying the optical slant-range diagram discussed 
in Section 5.2. For a variety of reasons, such data 
were never secured. Faulty calibration procedures 



Figure 53. Aerial photograph of 5-step gray scale 
used at Orlando, Florida. 


and/or mechanical failures of the K-24 cameras 
spoiled each attempt during the flights in Florida 
and California. These troubles were subsequently 
eliminated and, after very thorough preparations, 
several flights were made at Bedford, Massachusetts, 
through the courtesy of NDRC Section 16.1, during 
a two-week period. On every one of these flights, the 
visibility over the target was so poor that no usable 
photographs could be made. 

Reflectance Data 

The simplest and most accurate method for eval¬ 
uating the reflectance of a terrestrial object involves 
a direct comparison of the light reflected by the ter¬ 
rain with that reflected by each of a series of large 
horizontal gray panels (a gray scale) laid nearby. 
The spectral reflectance of each panel of the gray 
scale can be determined in the laboratory by means 
of a spectrophotometer. 


A gray scale of five 50x50-foot squares made up 
of painted panels of Celotex (Figure 53) was used 
for this type of experiment during a series of flights 
from the airbase at Orlando, Florida. On a day 
when the sky was completely free from clouds so 
that the gray scale and the surrounding countryside 
were lighted uniformly, the spectrogeograph was 
flown at constant altitude over the gray scale and 
over a variety of nearby terrains. After develop¬ 
ment, the spectrograms were measured with the den¬ 
sitometer described in Section 5.4.1. At a selected 
wavelength, the value of density corresponding to 
each step on the gray scale was plotted against the 
known spectral reflectance of the panel (Figure 54), 
and the reflectances of nearby terrains were read 
directly from this curve. This process was repeated 
at many wavelengths throughout the spectrum, and 



Figure 54. Typical plot of density of spectrogram vs. 
reflectance of gray scale at wavelength 520 millimi¬ 
crons. 


the resulting spectrophotometric curves of the ter¬ 
rains were plotted. Figure 55 shows a typical curve 
obtained in this manner. Other examples will be 
found in Figures 1 and 12 of OSRD Report No. 
6554. 14 This report, Reflectance of Natural Terrains , 
presents all the spectrophotometric and snectroradio- 
metric data obtained by the Tiffany Foundation in 
Florida and California; a copy of the report will be 
found in the microfilm supplement to this volume. 
The results of a colorimetric analysis of some of the 


CONFIDENTIAL 












210 


VISIBILITY FROM AIRCRAFT 


figures contained in the Tiffany report are shown 
in Table 1. 

An alternative procedure, involving only two 
panels of known reflectance, was also used in Florida 



WAVELENGTH IN MILLIMICRONS 

Figure 55. Spectral reflectance of green field. 

and in California for measuring the reflectance of 
natural terrains. On theoretical grounds, only two 
known panels are needed if the spectrogeograph is 
calibrated independently. 48 Techniques for calibrat¬ 
ing the instrument are discussed in detail in OSRD 
Report No. 6555, 15 and the data in all except Figures 
1, 12, and 29 of OSRD Report No. 6554 14 were ob¬ 
tained in this manner. 


Reflectance of Underwater Terrains. The reflect¬ 
ance of underwater terrains was studied with the 
spectrogeograph in quest of data which subsequently 
provided a basis for improved methods of aerial 
photographic reconnaissance for ocean shoals. This 
work was undertaken at the request of the Navy 
following the landings at Tarawa. Shoal waters off 
Dania, Florida, were specified by the Navy’s liaison 
officer to this research. Buoys (Figures 56 and 57) 



Figure 56. Floating buoy, number 4, being launched. 
Ten such buoys were anchored off the beach at Dania, 
Florida. 


were anchored at intervals along a line perpendicu¬ 
lar to the beach in order to provide positive identifi¬ 
cation of the water depths. Samples of the bottom 
were collected for spectrophotometric study. Two 


Table 1 . Color of natural terrains. (ICI illuminant C, standard observer, and coordinate system.) 50 


No. of fig. 

OSRD Report Subject 

No. 6554 

Y 

X 

V 

Dominant 

wavelength 

(m^i) 

Excitation 
purity 
(per cent) 

1 

Large maple tree (1) 

0.0614 

0.378 

0.433 

570.4 

49.5 


Citrus tree (2) 

0.0377 

0.357 

0.384 

572.9 

30.6 

2 

Dark green field 

0.0234 

0.350 

0.370 

573.7 

24.8 

5 

Green field with soil showing 

0.0269 

0.332 

0.362 

568.0 

18.1 

6 

Light green tree (1) 

0.0394 

0.338 

0.400 

564.3 

30.0 

9 

Yellow-green vegetation (1) 

0.107 

0.366 

0.410 

570.9 

40.0 

10 

Tree shadow on brown soil (2) 

0.0128 

0.299 

0.308 

481.5 

4.7 

12 

Asphalt paving (1) 

0.0313 

0.332 

0.330 

584.6 

9.9 


Ground with little vegetation (2) 

0.0844 

0.355 

0.352 

580.6 

21.8 


Sandy soil (3) 

0.103 

0.340 

0.345 

578.3 

15.7 

13 

Mud (3) 

0.0691 

0.390 

0.387 

579.6 

40.6 

14 

Pond 

0.0142 

0.277 

0.283 

477.5 

15.8 

15 

Red soil in California desert 

0.0842 

0.359 

0.347 

584.0 

21.3 

17 

Dry wash 

0.151 

0.346 

0.323 

601.0 

12.0 

18 

Light red ground (1) 

0.0652 

0.382 

0.355 

586.3 

29.7 

20 

Yellow sand dune (2) 

0.206 

0.400 

0.375 

583.7 

40.0 

21 

Salt flats 

0.197 

0.340 

0.327 

591.8 

10.7 

22 

Dark brown sand in dry wash 

0.168 

0.335 

0.326 

591.0 

9.2 

24 

Light sand on slope of volcano (1) 

0.150 

0.355 

0.352 

580.6 

21.7 


Dark volcanic rock (2) 

0.0569 

0.322 

0.320 

493.4 

4.1 

28 

Desert during green season 

0.0689 

0.362 

0.360 

579.9 

25.8 


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THE REFLECTANCE OF NATURAL TERRAINS 



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212 


VISIBILITY FROM AIRCRAFT 


large panels of known reflectance were placed on the 
beach, and the spectrogeograph was flown over the 
shoals. Figure 58 shows the resulting spectroradio- 
metric curves. 15 

Because color-changes with depth are partially 
masked by light reflected from the surface of the 
sea, the experiment was repeated with the spectro- 



WAVELENGTH IN MILLIMICRONS 

Figure 58. Spectroradiometric curves of the light 
reaching the spectrogeograph 4.000 feet above the 
shoal waters off Dania, Florida. 

geograph mounted in a glass-bottomed boat (Figures 
59 and 60). The resulting spectroradiometric curves 
are shown in Figure 61. The relation between water 
depth and color, as computed by standard proce¬ 
dures from the curves of Figures 58 and 61, is shown 
in Figure 62. Here the loci of the colors seen from 



Figure 59. Glass-bottomed boat in which spectrogeo¬ 
graph was mounted. 


the air and from the glass-bottomed boat are shown 
on the standard I.C.I. chromaticity diagram. 

It is clear from Figures 58 and 61 that greatest 
variations of reflectance with respect to depth occur 
in the spectral region from 540 to 570 millimicrons. 
The data and this conclusion were made known to 
the Navy through the office of the Coordinator of 



Figure 60. Spectrogeograph mounted in glass-bot¬ 
tomed boat. 



Figure 61. Spectroradiometric curves of the light 
reaching the spectrogeograph in the glass-bottomed 
boat from the shoals off Dania. Florida. 


CONFIDENTIAL 
















THE REFLECTANCE OF NATURAL TERRAINS 


213 


Research and Development. It is understood that the 
techniques for photographing ocean shoals were im¬ 
proved as a result of this information. 



t— 

Figure 62. Standard I.C.I. chromaticity diagram 
showing the loci of the colors of the ocean shoals as 
seen from the air and from the glass-bottomed boat. 

Texture 

The spectral reflectance of a forest (or a tree) 
differs from that of a leaf, partly because of the dark 
shadow pockets in which some of the light is trapped, 
and partly because of the interreflections between 
the leaves. This is illustrated in Figure 63, which 
shows the spectral reflectance of a tree as measured 
from aloft by the spectrogeograph, and the spectral 
reflectance of a leaf as measured by a spectropho¬ 
tometer. These curves differ because of the texture 
of the tree. 

Camoufleurs have long recognized the necessity 
of simulating the effects of natural texture. Nets 
festooned with colored garlands are commonly used 
for this purpose. Similarly, tufts of dyed fibers, 
chicken feathers, stone chippings, and many other 
“rough” materials have been employed. Research 
early in the war by the Passive Defense Project 
showed that the principal optical properties of a 
textured surface depend very little upon the size of 
the texturing elements but are controlled by the 
sharpness of the angles formed by adjacent struc¬ 


tures, that is, by the jaggedness of the profile of 
the surface. 103 

Texture-Simulating Paint 
Paint having a microscopically jagged surface 
(Section 1.3.6) was developed by the Interchemical 
Corporation under contract OEMsr-697, supervised 
by Section 16.3 of NDRC. Notable success was 
achieved in preparing a paint which simulated the 
texture and gloss characteristics of the wintertime 
deciduous forest, but attempts to make a corre¬ 
sponding paint to simulate a green forest were only 



Figure 63. Spectrophotometric curves. 

(a) of a maple tree as measured from aloft by the spectro- 
geograph, and (b) of a maple leaf as measured in the laboratory 
by a spectrophotometer. 

partially successful. Moreover, no blue, yellow, or 
green pigments were found which have the proper 
optical and chemical properties and adequate per¬ 
manence in sunlight. Because this development came 
when the need for defensive camouflage was dimin¬ 
ishing rapidly, research on the texture-simulating 
paints was interrupted and no Service application 
of them is known to have been made. 

5 ' 4 ' 4 Gloss 

Artificial texture seldom matches its natural sur¬ 
roundings from all points of view; camouflaged air- 


CONFIDENTIAL 











214 


VISIBILITY FROM AIRCRAFT 


strips and roof tops often stand out prominently 
when viewed up-sun. This is caused by a difference 
in gloss or gonioreflectance between the camouflage 
and the natural terrain. 

Research on the gloss characteristics of naturally 
and artificially textured surfaces was conducted by 
the Passive Defense Project before World War II. 
These studies showed the characteristic differences 
illustrated by the curves in Figure 64 to be responsi¬ 
ble for the visibility of flat surfaces coated with con¬ 
ventional matte paint when viewed up-sun. 

Goniophotometry 

The instrument used by the Passive Defense 
Project in its gloss studies was the photoelectric 
goniophotometer in the Illuminating Engineering 
Laboratories of the Massachusetts Institute of Tech- 



Figure 64. Goniophotometric curves (flux vs. angle) 
illustrating the characteristic difference between a 
matte surfaced paint and a textured surface. 


nology. 49 In this instrument, a sample approximately 
3 inches in diameter is illuminated at any selected 
angle of incidence by substantially collimated light. 
A photoelectric cell, carried by a movable arm 
pivoted about the center of the sample, measures 
the light reflected in various directions. A polar plot 
of the photocell readings produces a goniophoto¬ 
metric curve of the type shown in Figure 64. Re¬ 
search was handicapped by the slowness of the 
instrument; several hours were required to obtain 
and plot the data for each curve. 

Automatic Recording Goniophotometer. An auto¬ 
matic recording goniophotometer was designed and 
partially constructed by the Passive Defense Proj¬ 
ect. The instrument was subsequently completed 
by the electronics staff of the Research Laboratories 
of the Interchemical Corporation under contract 
OEMsr-697, and turned over to the Materials Lab¬ 
oratory of the Engineer Board, Corps of Engineers, 


Fort Belvoir, Virginia. With this instrument, shown 
in Figure 3, Chapter 1, a complete curve can be 
drawn in less than five minutes. The main frame of 
this instrument was cast from patterns loaned by the 
Massachusetts Institute of Technology and is thus 
identical with the frame of their goniophotometer. 
The photometric system is essentially identical with 
that of the densitometers described in Sections 1.3.3 
and 5.4.1 of this volume. Details of the electrical and 
optical system will be found in OSRD Report No. 
6556, 8a_e a copy of which appears in the microfilm 
supplement. It may be noted, however, that the 
source spread was made % degree in order to simu¬ 
late the geometry of sunlight. The sensitivity of the 
instrument depends upon the receiver spread. When 
this is made as great as is recommended by the 
American Society for Testing Materials, the instru¬ 
ment is so sensitive that a reflectance of 0.001 rela¬ 
tive to a perfect mirror can be made full-scale on 
the recording paper. The instrument will then bal¬ 
ance to within less than 1 per cent. This high sensi¬ 
tivity is required in order to measure the goniore¬ 
flectance of dark, textured surfaces. Some typical 
curves made by the recording goniophotometer are 
shown in Figures 65 and 66. 

Goniophotometry from the Air 

Provision has been made for adapting the spectro- 
geograph to gonio-spectrophotometric measurements 
of terrestrial targets by means of the mirror system 
shown in Figure 46. The target must be a uniform 
area of considerable extent ( 1 / 4- to ^-mile square) 
in order to allow' for uncertainties in the attitude of 
the airplane at the instant of exposure. 

The best flight plan for such an experiment is be¬ 
lieved to consist of a pattern of four courses, 90 
degrees apart, forming a square centered on the 
target and so oriented that two sides of the square 
are cross-sun; the remaining courses being up-sun 
and down-sun respectively. A series of square courses 
of various sizes and altitudes enable goniophoto¬ 
metric curves in the plane of incidence and perpen¬ 
dicular to it to be determined. The square courses 
should be corrected continuously for the apparent 
motion of the sun. This is accomplished very easily 
if the aircraft is equipped wfith photoelectric solar 
navigation equipment. Otherwise, the correction can 
be accomplished by carefully corrected compass 
bearings. A technique for navigation by compass 
during such experiments was worked out by the 
Army during the flights at Orlando, Florida, and it 


CONFIDENTIAL 












THE REFLECTANCE OF NATURAL TERRAINS 


215 




Figure 65. Typical curves drawn by the recording goniophotometer. 

(a) low-gloss paint, incident angle 30 degrees, range 0-10, no filter, % inch stop; (b) lusterless paint, incident angle 60 degrees, no 
filter, stop J /4 inch by 2 inches. 


is discussed in detail in OSRD Report No. 6555. 15 
Adverse weather conditions and technical difficulties 
frustrated both of the two attempts by the Tiffany 
Foundation to conduct an aerial goniophotometric 
experiment. 

Camouflage, to be effective from all points of 
view, must match its surroundings goniophotomet- 


rically. After a sufficient body of data concerning 
the goniophotometric properties of natural and arti¬ 
ficial terrains have been collected with the spectro- 
geograph, the gloss characteristics required of 
satisfactory camouflage materials can be specified 
in terms of the readings of a laboratory goniopho¬ 
tometer. 


CONFIDENTIAL 





































216 


VISIBILITY FROM AIRCRAFT 



Figure 66. Typical curves drawn by a recording goniophotometer. 

(a) emery paper, incident angle 60 degrees, no filter, no stop; (b) burlap, incident angle 60 degrees, no filter, stop % inch by 2 
inches. 



55 CAMOUFLAGE ENGINEERING 

The ultimate goal of the spectrogeograph program 
as originally conceived was the establishment of 
engineering procedure capable of predicting the 
allowable differences in reflectance between a cam¬ 
ouflaged object and its surroundings. There is almost 


no limit to the perfection with which it is possible 
to match natural terrain with artificial construction. 
However, the cost involved and the labor required 
increase so sharply as perfection is approached that, 
of necessity, camouflage measures far short of per¬ 
fection must be adopted. Indeed, the economic fac¬ 
tors are so critical that a slight relaxation in the 


CONFIDENTIAL 
































CAMOUFLAGE ENGINEERING 


217 


optical requirements may allow savings of many 
thousands of dollars in the cost of a camouflage 
treatment. However, such a relaxation of require¬ 
ments based upon guesswork may prove costly, for 
obviously camouflage is valueless unless it fulfills 
its purpose. The spectrogeograph program was un¬ 
dertaken in the hope of providing an engineering 
basis for camouflage design which would avoid 
costly empirical mistakes, while taking full advan¬ 
tage of the permissible tolerances afforded by the 
veiling blanket of atmospheric haze and the distance 
of the enemy. It is hoped and believed that the con¬ 
cepts, data, and nomographic charts contained in 
this volume fulfill the basic requirements, and make 
camouflage engineering a reality. 

°' 51 A Typical Problem 

As an example of a typical camouflage engineer¬ 
ing problem, consider the requirements to be met by 
the camouflage for a building having a flat, rectangu¬ 
lar roof, 100x400 feet, situated among dense decidu¬ 
ous trees, with the long dimension of the roof in the 
east-to-west direction. Let it be assumed that, ac¬ 
cording to military advice, attack is most likely to 
occur during midmorning hours in clear weather by 
bombers flying from the east at an altitude of 25,000 
feet. Let it be assumed further that the camouflage 
will have served its purpose if the target is invisible, 
despite identifying landmarks, until the plane is so 
nearly over the target that bombs cannot be dropped 
on it. 

The minimum distance from which a successful 
bombing run can be made depends on the character¬ 
istics of the bombsight, the ground speed Of the 
plane, and its altitude. For the purposes of this 
example, let it be assumed that, for a successful 
attack, the target must be visible to the bombardier 
when he looks along a sight path corresponding to 
0 = 40 degrees, as shown in Figure 67. If, at this 
critical point, the target is only liminally visible, 
the camouflage will have fulfilled its purpose and 
full advantage will have been taken of the available 
tolerances. 

Preliminary Calculations 

Before the nomographic visibility charts can be 
used, the following preliminary calculations are re¬ 
quired: 

Slant-Range. At the critical point the slant-range 
R must be calculated. The value is expressed as 


R = sj n 40° ~ 39j000 feet — 13 > 000 y ards - 

This value can also be obtained from the optical 
slant-range diagram (Figure 2) by noting the inter¬ 
section of the solid curve for 6 — 40 degrees with 
the dashed curve for altitude 25,000 feet. 

Optical Slant-Range. If the optical standard at¬ 
mosphere is assumed, the optical slant-range R is 
shown by Figure 2 to be 7,690 yards; if nonstandard 
atmospheric conditions are expected to prevail, a 
corrected value of R can be found by the procedures 
described in Section 5.2.1. 



Figure 67. Bomber at the critical point in its ap¬ 
proach to the target under conditions assumed in 
Section 5.5.1. 


Projected Target Area. The projected area of the 
target is: 

A = 100 X 400 X si n 40° = 25,700 square feet. 

Effective Projected Target Area. The appropriate 
value of effective projected target area A can now 
be found with the aid of the nomographic chart 
shown in Figure 31, or by substitution in equation 

a), 

/ 7 690 \ 2 

A = y j (25,700) = 9,060 square feet. 

Meteorological Data 

In addition to R and A, values of the sky-ground 
ratio and of the meteorological range must be had 
before the nomographic visibility charts can be used. 

The Sky-Ground Ratio. If the reflectance Y of the 
terrain surrounding the target is known from meas¬ 
urements made with the spectrogeograph or by some 
equivalent method, the brightness of the ground can 
be found by multiplying Y by the illumination on 
a horizontal plane at ground level. A variety of 
visual and photoelectric illumination meters (il- 


CONFIDENTIAL 











218 


VISIBILITY FROM AIRCRAFT 


luminometers) are on the market. Some of these in¬ 
struments are also capable of measuring the bright¬ 
ness of the horizon sky in the directions m and n 
shown in Figure 17, Chapter 2. The sky-ground 
ratio can then be found by dividing the brightness 
of the sky by the brightness of the ground. 

For the purposes of the present example, let it be 
assumed that the required measurements have been 
made and that a sky-ground ratio of 4.0 has been 
found for the conditions under which concealment 
is desired. 

The Meteorological Range. A photoelectric trans- 
missometer for measuring the meteorological range 
has been devised by the U. S. Bureau of Standards, 20 
and it is understood that a number of these instru¬ 
ments are in use throughout the country. A variety 
of other instruments for measuring v have been 
described in the literature. 12 When instrumentation 
is not available, the visibility (Section 2.2.5) can 
be estimated by eye in the manner customarily em¬ 
ployed by meteorologists. Such visibility estimates 
are usually included in aircraft weather reports. As 
pointed out in Section 2.2.5, the meteorological 
range should be taken as 4/3 times the visibility. 

Use of the Visibility Charts 

Selection of the Proper Chart. The selection of the 
proper visibility chart for use in a given problem 
is governed by the level of brightness to which the 
eyes of the observer are adapted and by the shape 
of the target. In the present example, full daylight 
brightnesses are involved. Since the sky-ground 
ratio is 4, the ground is one-quarter as bright as the 
sky in the directions m and n (Figure 17, Chapter 
2). However, as seen from above, the apparent 
brightness of the landscape is increased by space 
light, so that a nomographic visibility chart for full 
daylight (B H ^ 1,000 foot-lamberts) should be used. 

Since the roof of the target is a rectangle 400x 
100 feet, its side-to-side ratio is 4:1 when viewed 
vertically downward. Because of the foreshortening 
due to the oblique angle of view (0 == 40 degrees) 
from the critical point, the effective side-to-side 
ratio of the target is reduced to 2.5:1. As explained 
in Section 4.5.1, the best answer will be intermediate 
between the values given by Figures 6 and 15. Since, 
however, these values differ only by the order of 5 
per cent, the shape of the target is unimportant in 
comparison with other factors and either chart may 
be used. Since no target is more visible than a circu¬ 


lar target of equal area, the use of the visibility chart 
for circular targets (Figure 6) is preferable. How¬ 
ever, no generalization should be drawn from the 
foregoing example concerning the importance of 
target shape; its effect should be considered in con¬ 
nection with every problem. For example, had the 
long dimension of the building been in the north- 
to-south direction, the shape of the target would 
produce a four times greater effect than in the case 
considered above. 

Procedure. The effective projected area A of the 
target and the optical slant-range R at which it 
must be liminally visible determine a point on the 
nomographic visibility chart. This target point is 
indicated by the letter T on Figure 68. Place a 
straightedge across the chart in such a manner as to 
connect the appropriate value on the meteorological 
range scale with T. The position of the straightedge 
is shown by the dashed line on Figure 68, for the 
case of a meteorological range of 10 miles. Place the 
point of a pencil at the intersection of the dashed 
line and the right-hand vertical boundary of the 
chart. Rotate the straightedge about this point until 
it passes through the appropriate value on the sky- 
ground ratio scale. This position of the straightedge 
is indicated by the dotted line in Figure 68, for the 
case of a sky-ground ratio of 4. Read the value of 
liminal contrast C 0 from the contrast scale at the 
middle of the diagram. In this case, C 0 = 0.079. 

The Role of Atmospheric Haze 

The value of contrast, which will make the target 
liminally visible from the critical point, depends 
upon the value of meteorological range. This rela- 


Table 2 


Meteorological 


range 

Liminal 

(miles) 

contrast 

00 

0.014 

50 

0.020 

20 

0.035 

10 

0.079 

8 

0.122 

6 

0.25 

4 

1.01 

3 

4.10 


tion is illustrated by Table 2. It w T ill be noted that 
when the air is exceptionally clear (v—» m) the 
target is visible unless its contrast is less than 0.014. 


CONFIDENTIAL 








VALUES OF R (YARDS) 


CAMOUFLAGE ENGINEERING 


219 



(S3“im) 30NVd lVDI0010a0313^ 


CONFIDENTIAL 


TARGET SHAPE! VALUES OF R (YARDS) Bh^IOOO F OOT-L A MBE RTS 

CIRCLE (FULL DAYLIGHT) _ 

Figure 68. Nomographic visibility chart illustrating method of solution of typical camouflage engineering problem (see Section 5.5.1). The 
symbol T indicates the target 'point. (The original of this figure is a photostat which shows serious distortion and this figure must not, therefore, 
be used in the solution of problems. It is intended only to illustrate the manner of using Figure 6.) 










































































































































































































220 


VISIBILITY FROM AIRCRAFT 



0.9 — 
0.8 — 
0.7 — 
0.6 — 

0.5 — 

0.4 — 

03 — 


0.2 — 


0.1 — 
0.0 9 — 
0.0 8 — 
00 7 — 

0.0 6 — 


1 , 000.000 ^ = 


100,000 -== = 


0,000 ■ 


5,000 
3.000 
2,000 

1,000 -^ T - 1,000 

500 
300 
200 


1 00 

20 

I 0 

5 

4 

3 


1.5 

1.3 

1.2 

l.l 

1.05 

1.03 

1.02 


1 . 000.000 
500,000 
3 00,000 
200,000 

■100,000 

5 0.000 
30,000 
20,000 

- 10,000 


100 

50 

30 

20 

10 

5 

3 

2 

I 

0.5 

0.3 

0.2 

0.1 

0.0 5 

aos 

0.0 2 


“ 0.0 0 5 

— 0.0 0 6 

— 0.0 0 7 

— 0.0 08 

— 0.0 09 

— 0.0 I 


— 0.0 2 


— 0.0 3 

E- 0.0 4 

— 0.0 5 

— 0.06 

— 0.07 

— 0.0 8 

— 0.09 

— 0.1 


0.2 


— 0.3 

— 0.4 


0.0 5 — 
0.04 —; 

0.0 3 —| 

0.0 2 — 


0.0 I 
0.0 0 9 
OX) 0 8 
0.0 0 7 

0.0 0 6 

0.0 0 5 




1.0 i 

1.005 

1.003 

1.002 

1.001 

1.0005 

1.0003 

1.0002 

1.0001 

1.00005 

1.00003 

1.00002 

1.0000 I 


0.005 

0.00 3 
000 2 


0.000 5 
0.000 3 
0.000 2 

0.000 I 

000005 
0.0000 3 
0.0000 2 

OOOOOI 


OOOI 


1.000005 

1.000003 

1.000002 


0X500005 
0.000003 
0.00000 2 


1.00000 I 


0.000001 



Figure 69. Nomographic chart representing equation (1) Chapter 3. 

When equivalent achromatic contrast (C 0 ) and brightness contrast (C B ) are known, color contrast (Co) may be found by using chart 
as shown in left-hand key diagram. When C 0 and Cq are known, C B may be found by using chart as shown in right-hand key diagram. 
When brightness contrast and color contrast are known, equivalent achromatic contrast can be found by using chart in either manner. 


Such a low value of contrast can be attained only 
with great difficulty, and the maintenance of such 
perfect camouflage is almost impossible because of 
fading and/or change in the natural background. 


In other w r ords, in the case of so large a target, the 
camoufleur must depend upon haze to conceal the 
target. 

Tone-Down. When the meteorological range is less 


CONFIDENTIAL 









































PEACETIME APPLICATIONS 


221 


than four miles, a contrast greater than unity is re¬ 
quired to make the target visible from the critical 
point. A white roof, which may be several times as 
bright as the surrounding terrain, will be plainly 
visible, but a roof that is darker than its surround¬ 
ings will not be seen by the enemy no matter how 
black it may be. Therefore, an application of black 
paint, tar, or other black material to the roof will 
provide camouflage during hazy weather. This cam¬ 
ouflage measure is known as tone-down. 

Color Contrasts. Although color contrasts have 
little effect on the visibility of naval targets (Sec¬ 
tion 4.9.3), they are often not negligible in the case 
of objects on the ground, where the brightness dif¬ 
ferences may be small. 

If the neutral point, represented by the cross in 
Figure 39, Chapter 3, is plotted on Figure 41, Chap¬ 
ter 3, it will be seen that an equivalent achromatic 
contrast of 0.12 is produced by the color contrast of 
a black roof in green surroundings. Table 2 indicates 
that this contrast will render the target visible from 
the critical point whenever the meteorological range 
v exceeds 8 miles. This value of v has been called 
the tone-down limit. 

Whenever the meteorological range exceeds the 
tone-down limit for the target, colored camouflage 
must be used to achieve concealment. 

Camouflage Design 

After tables similar to Table 2 have been pre¬ 
pared for various values of sky-ground ratio, the 
type of camouflage to be used can be chosen on the 
basis of a compromise involving cost, frequency of 
occurrence of the various types of weather, and the 
military or economic value of the target. If the tone- 


down limit is to be approached or exceeded, a choice 
may be made between brightness contrast and color 
contrast. This choice will be governed by the cost, 
the availability, and the permanence of the camou¬ 
flage materials rather than by any optical prin¬ 
ciples. 

To aid the camoufleur in computing resultant 
achromatic contrast from the brightness contrast 
and the equivalent achromatic contrast, a conven¬ 
ient nomographic representation of equation (1), 
Chapter 3, is presented in Figure 69. 

After the reflectances of camouflaged objects as 
measured from the air with the spectrogeograph 
have been correlated with spectrophotometric and 
goniophotometric properties of the camouflage mate¬ 
rials as measured in the laboratory, suitable mate¬ 
rials can be selected. Special flights with the spec¬ 
trogeograph are required only when the natural 
terrain surrounding the target has not previously 
been catalogued. However, the spectrogeograph may 
be used whenever the importance of a target war¬ 
rants a final check on the performance of the com¬ 
pleted camouflage installation or when the visibility 
of an object on the ground is to be studied. 

56 PEACETIME APPLICATIONS 

The foregoing discussion of the visibility of ob¬ 
jects on the ground has been written from the point 
of view of the camoufleur. However, the methods 
and data presented in this volume make possible the 
prediction of the visibility of landmarks, landing 
fields, and hazards to aerial navigation. It is ex¬ 
pected that the principles discussed herein will find 
valuable peacetime applications in military, naval, 
c mmercial, and private aviation. 


CONFIDENTIAL 












PART III 


AIRCRAFT CAMOUFLAGE 










Chapter 6 

CAMOUFLAGE OF SEA-SEARCH AIRCRAFT 


61 INTRODUCTION 

W hen the menace of German submarines to 
Allied Atlantic shipping constituted one of the 
major problems of World War II, the Camouflage 
Section of NDRC was requested by the Director of 
Technical Services of the Army Air Forces to devise 
a method of camouflage which would enable a radar- 
equipped, sea-search aircraft 1 to approach a sur¬ 
faced submarine within 30 seconds of flying time, 
before the aircraft became visible to members of the 
U-boat crew. Such an approach would ordinarily 
enable the aircraft to release its depth charges 
before the submarine could execute a crash 
dive. 

The Director of Technical Services was informed 
that even a white airplane will ordinarily be seen 
as a dark silhouette against a sky background, and 
that, although the plane might be rendered invisible 
by floodlighting, the amount of power required would 
be prohibitive. It was indicated, however, that if the 
plane could always approach the submarine in such 
a manner as to present the same head-on aspect, 
concealment might be possible by placing lights 
along the leading edge of the wings and in the 
fuselage section. It is known from data on the visual 
acuity of the human eye that, at a distance of two 
miles, individual lights are indistinguishable as such, 
if their spacing is less than about four feet. If, by 
means of suitable reflectors, the light from each 
lamp is confined to a narrow beam visible only from 
the deck of the submarine, the most economical use 
of power is achieved. The Director stated that the 
plane could be flown on any required tactical course 
and, as a basis for calculation, it could be assumed 
to hold a course toward the submarine with a devia¬ 
tion of less than 3 degrees. It was calculated, on this 
basis, that even a bomber as large as a Liberator 
(Figure 1) could be made to match ordinary sky 
backgrounds with a power consumption of less than 
500 watts. 


a In classified correspondence, this project was referred to 
by the code name Yehudi. For the benefit of those un¬ 
familiar with this neologism, Yehudi symbolizes in contem¬ 
porary slang “the little man who wasn’t there.” 


62 PRELIMINARY EXPERIMENT 

Pursuant to instructions issued by the Chief of 
Section 16.3 of NDRC, the staff of the Tiffany 
Foundation started work on an experimental test 19 
of a new camouflage principle by which a black 
silhouette can be rendered invisible to an observer 
through the use of lamps adjusted to the proper 
intensity and directed toward the observer. For this 
experiment, 1 * a black-painted board 2 inches wide by 
32 inches long was provided at 4-inch intervals with 
lamp and reflector units taken from hand flashlight 
assemblies, as shown in Figure 2. Each unit was 
composed of a prefocused bulb operated at 2.4 volts 
and 0.5 amperes with a parabolic reflector giving a 
beam-spread of about 2 degrees. The plain glass lens 
of each reflector unit was opaqued over the greater 
part of its area to reduce its candle power. This left 
a horizontal strip *4 inch wide by 1% inches long, 
which was covered with a paint film composed of a 
transparent blue pigment dispersed in linseed oil. 
This film converted the spectral energy distribution 
of the tungsten lamps to approximately that of day¬ 
light. A rheostat was used to adjust the intensity of 
the lamps to a brightness-match with the sky back¬ 
ground. 

6,2 1 Demonstration of the Principle 

A demonstration of the Yehudi principle was ar¬ 
ranged for Service representatives. The model was 
fixed horizontally between two vertical supports, 
which were mounted on the roof of the studio build¬ 
ing 50 feet above the ground, and was so adjusted 
that the beams of the lamps converged at a point 
900 feet distant. The viewing range lay in a north- 
south direction with the observation point at the 
southern end. 

In this demonstration, held on a clear day between 
10:30 a.m. and 12:30 p.m., Eastern war time, the 
model was boldly silhouetted against the northern 
sky. When the lamps were switched on, the model 

b Sections 6.2 through 6.6.9 are reproduced from OSRD 
Report No. 3816, Camouflage of Sea-Search Aircraft (The 
Yehudi Project ), 19 by the Louis Comfort Tiffany Founda¬ 
tion, Oyster Bay, New York, June 1, 1944. under Contract 
No. OEMsr-597. 


CONFIDENTIAL 


225 




226 


CAMOUFLAGE OF SEA-SEARCH AIRCRAFT 



Figure 1. Artist’s conception of a Liberator (B-24) camouflaged for sea-search in accordance with the Yehudi 
principle. 

In this application, lights of the sealed-beam type are shown mounted in the leading edges of the wings, in brackets beneath the 
wings, and in brackets around the fuselaee. 

The tests described in this report show that, when treated with this camouflage measure, a Liberator (B-24) can be rendered invisible, 
even under perfect weather conditions, at ranges as short as 30 seconds of flying time. 



Figure 2. Details of experiment designed to demon ;trate the Yehudi principle. 


CONFIDENTIAL 




















FULL-SCALE SILHOUETTE OF LIBERATOR 


227 



Figure 3. Map of Oyster Bay and vicinity. 

The dashed line indicates the 13,000-foot range from the observing station on the left to the hilltop station on the right. 


became invisible, even though the vertical supports 
served to fix its location. Light clouds appeared in 
the sky during the period of the demonstration, and 
appropriate adjustments were made in the lamp 
current to maintain a brightness-match. The effect 
of operating the lamps above and below the best 
value for the existing sky brightness was also dem¬ 
onstrated. It was noted that enough reserve power 
was available to match the brightness of a white 
card exposed in full sunlight behind the model. As 
a result of this demonstration, a decision was made 
to construct a full-scale silhouette of a Liberator 
(B-24), to suspend it from steel towers to be erected 
on the estate, and to observe the effectiveness of the 
Yehudi principle from an observing station two 
nautical miles distant. 

63 FULL-SCALE SILHOUETTE OF 
LIBERATOR 

The choice of a site for testing the full-scale 
model was dictated, primarily, "by the following con¬ 


siderations: (1) the necessity of securing the large- 
scale experiment against unauthorized observation, 
(2) the desirability of making observations over 
water, and (3) the requirement that the model be 
elevated as high as possible. A study of the terrain 
in the Oyster Bay region revealed that a ridge 180 
feet high on the Tiffany property would provide ade¬ 
quate height and that the model should be visible 
from the shore of Oyster Bay from a point approxi¬ 
mately 13,000 feet distant. Such a range would be 
about 85 per cent over water, with the eastern sky 
as background. The viewing station selected was 
a semicircular parking area on the shore road 
a few feet above the water at high tide (Fig¬ 
ure 3). 

The ridge on which the towers were to be erected 
was accessible by a dirt road but was heavily 
wooded. To provide an unobstructed view of the 
silhouette from the viewing station, it was necessary 
to clear several acres of trees and underbrush. Com¬ 
munication between the hilltop station and the shore 


CONFIDENTIAL 














228 


CAMOUFLAGE OF SEA-SEARCH AIRCRAFT 


station was arranged by installation of a private 
telephone line. 

6 - 31 The Silhouette 

Only the head-on aspect of a Liberator was in¬ 
volved in this experiment, and the first step was to 
reproduce the corresponding silhouette. Manufac¬ 
turer’s drawings were not readily available for this 
purpose, but the necessary data were obtained from 


IIP -O" 



Figure 4. Silhouette of Liberator in head-on aspect. 


a photograph of a wind-tunnel model supplied by 
Wright Field. Additional details were obtained from 
an actual photograph of a Liberator taken on the 
ground. The silhouette drawing used is reproduced 
in Figure 4. 

A model was constructed of plywood and rein¬ 
forced to provide sufficient rigidity. 


6.3.2 Method of Suspension 

Two steel towers, 100 feet high, were erected 200 
feet apart, and the model was supported on a 
i/ 2 -inch steel cable between them. This cable was at¬ 
tached to winches at the base of each tower; and 
the model was raised from its resting place in cradles 
on the ground to an elevation of 85 feet above the 
ground by simultaneous operation of the two 
winches. Guy wires were rigged to steady the model 
in the elevated position. The upper half of the steel 
towers was painted white to reduce the contrast 
when seen against the sky. When in the elevated 
position, the model was 235 feet above sea level. A 
view of the elevated silhouette is shown in Figure 5 
and the rigging layout in Figure 6. It is of interest 
that, on the occasion of its first elevation, the ap¬ 
proach of a four-motored bomber was reported by 
the local volunteer airplane spotters several miles 
away. 

6.3.3 Arrangement and Control of Lights 

It had been calculated previously that 500 watts 
of power would suffice for this experiment. Special 



Figure 5. Close-up view of suspended silhouette. 


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FULL-SCALE SILHOUETTE OF LIBERATOR 


229 



Figure 6. Rigging layout for the suspension of the full-scale model. 


lamps were supplied by the General Electric Com¬ 
pany, Cleveland, Ohio. These lamps were of the 
sealed-beam type, 4 inches in diameter, with a single¬ 
coil filament operating at 6.5 volts and 1.7 amperes. 
They had a beam-spread of 3 degrees in the hori¬ 
zontal plane and 6 degrees in the vertical plane. The 
lamps were wired two in series, with pairs in paral¬ 
lel, and were operated from the 18-volt secondary 
of a transformer supplied from a portable 110-volt 
a-c generator of 500 watts capacity. The intensity 
of the lamps was adjusted by means of a Variac in 
the primary circuit of the transformer, a voltmeter 


across the secondary circuit being used for reference. 
The clear glass face of each lamp was coated with 
a transparent paint containing iron-blue (red shade) 
to correct the light to daylight quality. 

The lamps were mounted in adjustable wooden 
frames which were clamped to tracks mounted on 
the face of the silhouette as shown in Figure 7. 
These tracks permitted easy adjustment of lamp 
positions. 

The method by which the proper spacing of the 
lamps can be calculated is discussed in Section 6.6. 
The arrangement shown in Figure 8 is one of several 



in* 7n.c&s 


Figure 7. Lamp details and method of mounting. 


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230 


CAMOUFLAGE OF SEA-SEARCH AIRCRAFT 



used during these experiments. Although determined 
somewhat empirically, it is a close approximation 
to the theoretical spacing. 

6 4 EXPERIMENTS WITH THE FULL-SCALE 
MODEL 

Because of the narrow beams of the lamps, the 
lamp alignment was correspondingly critical. With 
the model in the elevated position, only overall ad¬ 
justments could be made and these were by use of 
the guy ropes. The necessary instructions were re¬ 
layed by telephone from the observers’ station. 
When the best overall setting had been obtained, a 
12-power telescope was used to inspect the align¬ 
ment of individual lamps. The model was then 
lowered and the individual lamp mounts were ad¬ 
justed by means of the thumb screws shown in 
Figure 7. The voltage across the lamps was then 
regulated on the basis of information from the ob¬ 
servers’ station until the minimum visibility was 
obtained. This voltage was found not to be especially 
critical. 

The experiments necessary for the determination 
of the proper spacing of the lamps and the necessary 
range of intensity and color were performed during 
the winter of 1943. During the greater portion of 
this period, the visibility was less than 2 miles; and 
clear weather was usually attended by winds of 
high velocity. Since the silhouette was located on 
the top of a ridge and presented approximately 300 
square feet of surface area, even winds of moderate 
velocity made elevation of the model a precarious 
undertaking. 

The most successful demonstration was made in 
the presence of four qualified observers. The visibil¬ 


ity was so high on this occasion that the 1-inch 
cables supporting the towers could be discerned 
without difficulty from the observing station. Never¬ 
theless, all observers agreed that the silhouette was 
completely invisible when the lamps were adjusted 
to the proper intensity. 

Mention should be made of the assistance rendered 
by the Police Department of Oyster Bay in connec¬ 
tion with these experiments and demonstrations. 
Patrol cars, furnished whenever requested, prevented 
automobiles from slowing down or stopping near 
the observation station. Because of the narrow hori¬ 
zontal beam-spread of the lamps (3 degrees), the 
alternate appearance and disappearance of the sil¬ 
houette during demonstrations could be witnessed 
only from points along the shore road within 300 
feet of the station. The observation station is de¬ 
picted in Figure 9. 

65 APPLICATION TO OTHER TYPES 
OF PLANES 

While the work on the full-scale model was in 
progress, a need arose for portable equipment suit¬ 
able for demonstrations of the Yehudi principle at 
several conferences in Washington. This need was 
finally met by an oil painting on thin sheet-metal, 
a photograph of which is shown in Figure 10. The 
black silhouette of a Liberator (B-24) was 3.75 
inches long in the painting. Hence, when viewed 
from a distance of 30 feet, the silhouette subtended 
the same angle as a Liberator at two statute miles. 

Small holes were drilled through the painting at 
points corresponding to the positions of the lights 
that had been calculated for the Liberator. A strip 
of onionskin paper was attached to the back of the 


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APPLICATION TO OTHER TYPES OF PLANES 


231 




Figure 9. Observation station of the viewing rarge during demonstration of the full-scale model. 

The silhouette is visible above the ridge, slightly to the left of the most distant boat. The telephone connection with the hilltop 
terminates in the box on the pole. 


painting to cover the holes, as shown in Figure 11 A. 
With a desk lamp placed behind the painting, the 
light issuing from the holes closely simulated the 
appearance of the lamps of the full-scale model. The 
intensity of the light issuing from the holes was ad¬ 
justed to the required level by varying the distance 
of the desk lamp from the back of the painting. 

Unless marked differences in color were present, 
the black silhouette could be made to disappear 
completely when the painting was viewed from the 
scale distance in the manner depicted in Figure 11B. 
An attempt to illustrate this demonstration by pho¬ 
tographic means is shown in Figures 12 and 13. 
These illustrations were reproduced from unre¬ 
touched negatives made with an ordinary view cam¬ 
era, a small stop in front of the lens being used to 
reduce its resolving power to correspondence with 
that of the human eye. Figure 12 is a photograph 



Figure 10. Oil painting of Liberator (B-24) used to 
demonstrate the Yehudi principle. 


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CAMOUFLAGE OF SEA-SEARCH AIRCRAFT 



Figure 11. (a) Desk lamp positioned behind the painting to illuminate the holes in the silhouette, (b) Observer 

viewing the painting at a distance of 30 feet (scale distance of 2 miles). 



Figure 12. Photograph taken with lens having re¬ 
solving power of human eye. 

When viewed at 11 feet, this photograph shows the 
appearance of a Liberator at 2 statute miles. 

made under these conditions before the desk lamp 
was turned on; Figure 13 is a photograph made un¬ 
der identical conditions after the desk lamp had 
been turned on. When these photographs are viewed 
from the normal reading distance (10 inches), they 



Figure 13. Photograph taken under the same condi¬ 
tions as Figure 12, with Yehudi camouflage applied. 

Note that, although the arrangement of lights is not perfect, 
the Liberator becomes invisible well within the proper viewing 
distance of HV 2 feet. 

represent the appearance of a Liberator only 880 
feet away. 

It was subsequently found that the illusion created 
is almost as realistic when a simple drawing is sub¬ 
stituted for the colored oil painting. In fact, the 


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TACTICAL AND TECHNICAL ASPECTS 


233 


drawing need be no more than a black paper sil¬ 
houette pasted on a large piece of white cardboard. 
This simplified technique has been used on several 
occasions to test the effect of modifying the lamp 
positions. For example, before an actual installa¬ 
tion was made on a Liberator at Wright Field, it 
was proposed that the lamps be mounted on brack¬ 
ets beneath the wings instead of in the leading edge 
of the wings. When a drawing of this arrangement 
had been made, it was seen at once that, although 
the underwing brackets would be satisfactory where 
the wing is thin, they would not properly camouflage 
the thicker portions of the wing structure. A new 
drawing, in which only the lamps near the wing tips 
were mounted on underwing brackets, showed that 
such an installation should be satisfactory. 

This technique's applicable to any type of plane 
and eliminates the necessity for constructing a 
model. Because atmospheric haze is absent, a design 
that performs satisfactorily at reduced scale can be 
expected to perform even more satisfactorily at full 
scale. Although this technique may be used for em¬ 
pirical determinations of lamp distribution, more 
direct methods will be discussed in Section 6.6. 

6 6 TACTICAL AND TECHNICAL ASPECTS 

The foregoing sections of this report are concerned 
with one specific tactical use of the Yehudi principle. 
The technical information that follows is supplied, 
however, because of other possible tactical uses. 
An attempt has been made to arrange the sub¬ 
ject matter of this section in the sequence that is 
normally employed in completing a camouflage de¬ 
sign based on the Yehudi principle. 

6.6.i Theoretical Power Requirements 

Assume a black aircraft to be viewed against a 
sky background of brightness B (candles per square 
foot). If the area of the silhouette (in square feet) 
is A, the equivalent intensity of the part of the sky 
obscured by the silhouette is BA (candles). Thus, if 
the sky has a brightness of 500 candles per square 
foot (500 jt foot-lamberts), and the area of the sil¬ 
houette is 200 square feet, the intensity of the por¬ 
tion of the sky obscured by the aircraft is 500 X 
200 = 100,000 candles. Apart from the fact that a 
single source does not provide the proper distribu¬ 
tion of intensity, a searchlight with a beam candle 
power of 100,000 would fulfill the requirements. If 


20 lights of equal intensity are to be employed, each 
should have a beam candle power of 5,000 candles. 

If the solid angle of the cone of light from each 
lamp is represented by S, the amount of flux asso¬ 
ciated with a total beam candlepower BA is BAS 
(.lumens), assuming the intensity to be uniform 
within the beam. Thus, if the total beam candle- 
power required is 100,000, and the solid angle is 0.02 
steradians, the number of lumens required is 100,000 
X 0.02 = 2,000 lumens. The luminous efficiency of 
tungsten lamps of the sealed-beam type is of the 
order of 20 lumens per watt at their normal operat¬ 
ing temperature. This means that 2,000 lumens can 
be supplied with a power expenditure of 2,000/20 = 
100 watts. 

Generalizing, the amount of power that is theo¬ 
retically required by a Yehudi installation is given 
by the following equation: 

p _BAS 

LT 

where Pis the power requirement in watts, 

B is the brightness of the sky in candles per 
square foot, 

A is the area of the silhouette in square feet, 

S is the solid angle in steradians; 

L is the efficiency of the lamps in lumens per 
watt, and 

T is the transmission factor of the color- 
correcting filters. 

The solid angle of a circular cone is related to the 
half-plane angle by the equation S = 2ji(1 — cos 6 ). 
Thus, for a circular cone whose plane angle is 
10 degrees, the solid angle is 

S — 2jt(1 — cos 5 degrees) = 0.0239 steradians. 

The shape of the filament commonly employed in 
lamps of the sealed-beam type is such that the cone 
of light is nearly rectangular in cross section. The 
solid angle of such a beam can be computed with 
sufficient accuracy in terms of the product of the 
two plane angles in radian measure; thus, a rec¬ 
tangular cone subtending 8 degrees in the vertical 
plane and 10 degrees in the horizontal plane repre¬ 
sents a solid angle of 10/57.3 X 8/57.3 = 0.0244 
steradians. 

6 . 6.2 Practical Power Requirements 

It was tacitly assumed in the foregoing that lamps 
can be obtained whose intensity is uniform within 


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234 


CAMOUFLAGE OF SEA-SEARCH AIRCRAFT 


the beam and is zero outside the beam. Lamps hav¬ 
ing this distribution of intensity are impossible of 
attainment, but lamp manufacturers are usually 
willing to supply data from which the applicability 
of their products for this purpose can be determined. 
Ordinarily, the catalogue description of sealed-beam 
lamps indicates the rated power input and the cor¬ 
responding candlepower in the center of the beam. 



Figure 14. Spectral transmittance curves of 6 differ¬ 
ent thicknesses of transparent, iron-blue, lacquer films 
applied to glass filters. 


Information concerning the solid angle of the beam 
and the criterion used in specifying the solid angle 
is frequently lacking. On inquiry, however, manu¬ 
facturers are usually willing to state the angular 
distribution of intensity; and it is common practice 
to define the spread of the beam in terms of the 
horizontal and vertical angles at which the inten¬ 
sity becomes some stated fraction of the intensity 
in the center of the beam. 

Emphasis has been placed on the sealed-beam 
type of lamp because it is so eminently suited for 
use in this connection. The reflector is of excellent 
quality and is made in a variety of standard sizes. 


The mounting of the filament is exceedingly sturdy 
and avoids the focusing difficulties associated with 
the old-fashioned type of headlamp with separate 
bulb and reflector. Although departure from one of 
the standard sizes is not justified unless purchase in 
considerable quantity is contemplated, special fila¬ 
ments can readily be incorporated in the standard 
envelopes. Because the volume of the envelopes is 
so much greater than that of the old-fashioned 
headlight bulb, many of the former limitations on 
filament construction are removed. The manufactur¬ 
ers of sealed-beam lamps have now had sufficient 
manufacturing experience with this type of unit 
to be able to design and produce, literally over¬ 
night, special lamps meeting specified require¬ 
ments. 

6 ' 6 ' 3 Color Correction 

Even when the candlepower of the Yehudi lamps 
is correctly adjusted for an intensity match with 
the sky, the airplane may be visible by virtue of a 
color difference. The color of a white cloud in direct 
sunlight is in the neighborhood of 5500° K, an over¬ 
cast sky has a color temperature approximating 
6500° K, and the color temperature of a blue sky 
may exceed 20000° K. Since tungsten lamps nor¬ 
mally have a color temperature in the vicinity of 
3000° K, it is common practice to increase their 
color temperature by the use of filters. As a matter 
of convenience and expediency, the filters used in 
the later experiments were made by coating glass 
plates with a film of clear lacquer containing a 
transparent iron-blue pigment (red shade). Figure 
14 shows the spectral transmittance curves of typi¬ 
cal filters produced by this method. The perform¬ 
ance and efficiency of these filters is indicated in 
Figure 15. 

Since the amount of light absorbed by the filter 
increases with the amount of color correction ef¬ 
fected, it is in the interest of power conservation to 
make no greater correction than is required for 
satisfactory performance. Presumably, filters made 
of colored glass or of colored plastic might be speci¬ 
fied for a large-scale installation, and it should be 
noted that many blue glasses and blue plastics trans¬ 
mit freely at the long wavelength end of the spec¬ 
trum. When used with incandescent lamps, this high 
red transmittance would make red goggles an effec¬ 
tive countermeasure. 


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TACTICAL AND TECHNICAL ASPECTS 


235 



Figure 15. Section of standard I.C.I. chromaticity diagram 50 * 51 showing the efficiency of the 6 filters whose spec¬ 
tral transmittance curves appear in Figure 14. 

The values of Y„ give the integrated transmittance for illuminant A, a tungsten source at 2848 degrees K. 


6 6 4 Number and Distribution of Lamps 

The resolving power of the human eye for two 
light sources of equal intensity is ordinarily as¬ 
sumed to be one minute of arc. To make certain that 
this value is of the correct order of magnitude under 
these special conditions, the following experiment 
was conducted in the vision range of the Tiffany 
Foundation. Observers viewed a uniformly illumi¬ 
nated white screen on which a black disk with a 
small central hole was mounted. It was found that, 
by properly adjusting the intensity of a lamp 
mounted behind the central hole in the black disk, 
the disk became invisible when it subtended an 
angle of less than 1.4 minutes at the eye of the 
observer. Photometric measurements confirmed that 
the candlepower supplied by the small lamp corre¬ 


sponded to the candlepower of the area of the 
screen obscured by the black disk. This experiment 
indicates that, even in a perfectly clear atmosphere, 
a black circular area 4.92 feet in diameter should 
be completely obscured at a distance of two nautical 
miles under the best conditions of observation when 
a source of the proper intensity is mounted at the 
center of the area. Subsequent experiments with the 
full-scale model were in accord with this calculation 
when applied to the fuselage section, but a greater 
spacing was found to be permissible along the thin 
sections of the wings. 

Within the limitations set by the resolving power 
of the eye, there is considerable latitude in the 
choice of number and distribution of the lamps. For 
example, if the spacing of the lamps is to be uniform, 
lamps of various candlepowers could be used. Such 


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LENCTH IN FEET 


Figure 16. The spacing of the lamps can be determined theoretically by dividing the total area into elemental 
areas equal in number to the number of lamps to be used on this half of the plane. From the standpoint of hori¬ 
zontal spacing, the lamps should be placed at the center of each elemental area as shown. 


an installation is impractical from many stand¬ 
points, and an equivalent result is secured by em¬ 
ploying lamps of equal intensity with appropriate 
spacing. A method for determining the appropriate 
spacing is illustrated in Figure 16. Structural con¬ 
siderations often preclude the mounting of lamps in 


the calculated locations, but minor displacements of 
the lamps are usually permissible, as can be demon¬ 
strated by the method outlined in Section 6.5 of this 
report. For example, it has been found by this 
method that although lamps suspended on brackets 
beneath the wing function properly near the wing 



Figure 17. Lamps suspended on brackets beneath 
outer wing. 


Figure 18. Lamps mounted in leading edge of in¬ 
board wing. 


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TACTICAL AND TECHNICAL ASPECTS 


237 


tips, they should be mounted nearer the center of 
the section wherever the wing section is thick. (See 
Figures 17 and 18.) In the fuselage section and 
around the motor cowlings, aerodynamic considera¬ 
tions may require that two or more lamps be re¬ 
placed by a single unit of correspondingly greater 
intensity. 

Alignment of Lamps 

The alignment of the lamps on an actual airplane 
should present none of the difficulties encountered 
in the final adjustments that were necessary on an 
elevated nonrigid structure. There are many pos¬ 
sible procedures, and an outline of one will suggest 
many variations. If a level airfield a mile or more 
in length is available, the airplane may be stationed 
at one end with its tail elevated on jacks to the 
proper attitude. By lighting one lamp at a time, an 
observer at the opposite end of the field can indicate 
the necessary adjustments. Portable radio equip¬ 
ment is useful in this connection. When all the 
lamps are in approximate alignment, a delicate test 
for horizontal adjustment can be made by noting 
whether all lamps remain equally bright when 
viewed from positions at equal lateral distances 
from the axis. The corresponding test for vertical 
adjustment can be made by raising or lowering the 
tail of the plane. Some time might be saved in the 
above procedure by making a preliminary adjust¬ 


ment by the method used in aligning automobile 
headlights, but this method is not sufficiently criti¬ 
cal for the final adjustment. 

6 6,6 Method of Controlling Intensity 

Throughout the experiments herein reported, pho¬ 
toelectric equipment has been used as a guide in 
controlling the intensity of the lights. Two photo¬ 
cells of the photronic type were employed in the 
bridge-type circuit shown in Figure 19. One of 
the cells is illuminated by the sky background and 
the other by an auxiliary lamp in the main lamp 
circuit. With this arrangement, a zero-center mil- 
liammeter gives no deflection when the two cells 
are equally illuminated. A Polaroid shutter is pro¬ 
vided between the auxiliary lamp and the photocell 
to adjust the zero position of the meter after the 
proper intensity has been found for one condition. 
Subsequent changes in sky brightness, as observed 
by the sky photocell, can be compensated by ad¬ 
justing the rheostat in the lamp circuit until the 
meter again reads zero. Such a manual control con¬ 
templates that some member of the crew will main¬ 
tain this adjustment prior to and during the attack. 
Full automatic control is possible by the use of a 
suitable servo-mechanism. 

Because of local variations in the brightness of 
the sky, the field of view of the sky photocell should 
theoretically be restricted to the angular divergence 



Figure 19. Photoelectric device used in the Tiffany experiments for matching the sky background. 


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238 


CAMOUFLAGE OF SEA-SEARCH AIRCRAFT 


of the light beam. It can be shown that the greatest 
quantity of flux is received by the photocell when 
the limitation of its field of view is accomplished by 
mounting an aperture identical in size with the 
photocell window at such a distance from the cell 
that the desired field of view is obtained. A greater 
quantity of flux and a resulting increase in the 
sensitivity of the control system can be achieved 
by replacing the front aperture with a lens having 
its focal point at the cell. When this is done, the 
field of view is independent of the lens diameter, 
and a gain of 36/(//number) 2 in sensitivity is se¬ 
cured. Suitable lenses having a relative aperture of 
//2 are marketed as reading glasses. With such an 
j/2 lens, a ninefold gain in flux results. When a 
collection lens is not used, the photoelectric control 
is forced to operate near its threshold sensitivity on 
dark days. 

6 6 ~ Color of Paint 

A black silhouette was used throughout the ex¬ 
periments described in this report because the in¬ 
tensity required to match the sky background is then 
independent of the illumination falling on the face 
of the silhouette. In an actual installation, the por¬ 
tions of the airplane that an enemy observer can see 
during a tactical approach may advantageously, 
from the standpoint of power requirements, be 
painted some other color. There is little benefit to 
be gained from painting the undersurfaces with a 
highly reflecting paint, since these surfaces receive 


very little illumination by reflection from the water 
and by scattering in the intervening atmosphere, 
but considerable saving in power can be effected by 
using a more highly reflecting paint on the vertical 
and top surfaces. However, the control of the light¬ 
ing equipment is then more complicated, because 
the illumination of the vertical surfaces varies enor¬ 
mously with the angle at which sunlight strikes 
them. 

If the important areas visible to an enemy ob¬ 
server are painted a dark, saturated blue, it should 
be possible to reduce the power requirements some¬ 
what without necessitating a more complicated con¬ 
trol mechanism. In this case, the light reflected from 
such surfaces will be predominantly blue; and blue 
light has little effect on luminosity. Blue light does 
increase the color temperature considerably, and the 
greatest increase would occur on sunny days when 
the sky background is most likely to be at the high 
color temperature of blue sky. 

668 The Effect of Crosswinds 

During discussions of this project with Service 
personnel, attention has often been called to the 
fact that, when crosswinds are encountered, air¬ 
craft would not ordinarily present their head-on 
aspect during an approach. Thus, even if the spread 
of the beams was great enough to include the target, 
the match with the sky would be imperfect because 
of the change in size and shape of the silhouette. 
Figure 20 shows the silhouette of a Liberator (B-24) 



Figure 20. (Above) Silhouette of Liberator viewed from the left at an angle of 20 degrees. (Below) Head-on 
silhouette. 


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CAMOUFLAGE OF NAVY SEA-SEARCH AIRCRAFT 


239 


at a heading of 20 degrees in comparison with one 
viewed head-on. Obviously, an installation of lights 
intended to camouflage over so wide an arc would 
require that lamps be mounted along the sides of 
the fuselage. Such complications are avoided if 
pilots are instructed to “home” on the target. In 
this case, the approach will be along the slightly 
curved course illustrated in Figure 21. 


6 7 CAMOUFLAGE OF NAVY SEA-SEARCH 
AIRCRAFT 

In 1943, the Aircraft Anti-Submarine Warfare 
Development Detachment of the Air Force, U. S. 
Atlantic Fleet [ASDevLant] stationed at the Naval 
Air Station, Quonset Point, Rhode Island, requested 
the aid of Section 16.3 of NDRC in connection with 



Figure 21. Diagram showing the curved flight course that results when the light beams are held on the target dur¬ 
ing an approach with a strong crosswind. 


This curve was plotted for the case of a crosswind whose velocity is one-fifth of the air speed of the plane. 


e.6.9 Xlie Effectiveness of Enemy 
Countermeasures 

The only effective optical countermeasure sug¬ 
gested thus far is the use of binoculars. Theoreti¬ 
cally, if there were no atmosphere, a perfect pair of 
8-power glasses would increase by eightfold the 
effective range of the Yehudi camouflage. Because 
light is always scattered to some extent by the 
atmosphere, the effectiveness of binoculars is always 
less than the theoretical value. Furthermore, the 
field of view of such binoculars is scarcely more 
than 5 degrees, which greatly increases the diffi¬ 
culty of search. At the time this project was started, 
it was understood that the Germans were using three 
observers on the decks of their submarines, each 
being assigned to search the sky through an arc of 
120 degrees. It was stated as part of the original 
project assignment that the use of this camouflage 
measure would be fully justified if its only result 
was to require that enemy lookouts use binoculars 
continuously. 

The use of color filters by enemy observers would, 
of course, be futile unless there is a marked spec¬ 
tral dissimilarity between the lights and the natural 
background which they attempt to simulate. Under 
certain special conditions, polarizing devices would 
increase the visibility of the camouflaged airplane, 
but these special conditions do not often occur. Sea- 
search planes equipped with Yehudi camouflage can 
be detected by enemy radar if the enemy is willing, 
or finds it necessary, to sacrifice radar silence. 


Yehudi camouflage. They wished to install lights on 
their patrol aircraft similar to those designed for 
the B-24 but intended for continuous use while fly¬ 
ing in clear weather under blue skies. For this pur¬ 
pose, the Navy requested a much greater horizontal 
beam-spread and the ability to match sky bright¬ 
nesses up to 1,500 foot-lamberts. Such a design was 
produced for the PBM flying boat. Later, a second 
design was undertaken at the Navy’s request which 
provided for intermittent operation with a beam- 
spread limited to 9 degrees and the matching of sky 
brightnesses up to 2,000 foot-lamberts. Although 
special sealed-beam lamps and the necessary hous¬ 
ings were devised, no installation was made on a 
PBM. 

In the same year, the ASDevLant Group at the 
Naval Air Station, Quonset Point, Rhode Island, 
requested aid in designing Yehudi camouflage for a 
TBF torpedo bomber. Such a design was made, and 
advice was given on several incidental problems. 
Navy photographs of the completed installation are 
shown in Figures 22 and 23. The first flight test 
occasioned favorable reaction. Further test flights 
resulted in improvements in the adjustment of the 
equipment and in techniques for its use. It is under¬ 
stood that under conditions such that an uncamou¬ 
flaged plane was visible at about 12 miles, the plane 
equipped with Yehudi camouflage could approach 
to within 3,000 yards without detection, even when 
its approximate location was indicated by an ac¬ 
companying uncamouflaged plane. 

On the basis of the recommendations in a report 


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240 


CAMOUFLAGE OF SEA-SEARCH AIRCRAFT 



Figure 22. TBF torpedo bomber equipped with Yehudi camouflage designed to conceal the aircraft from visual 
detection by an observer at the target until the range has been closed to 1.5 miles. 

In Navy tests, the camouflage fulfilled this requirement even on days so clear that the plane was visible at 12 miles when the 
Yehudi lamps were not lighted. 



Figure 23. Side view of the TBF torpedo bomber shown in Figure 22. 

The Yehudi lamps are contained in housings protruding from the leading edge of the wing, and in tapered mounts on the motor 
cowling. 


issued by ASDevLant, ComAirLant ordered the in¬ 
stallation of Yehudi camouflage on an operating 
squadron of TBF aircraft. Concurrently, the Naval 
Air Station at Patuxent River, Maryland, was asked 


to make the necessary changes in the engineering 
drawings of the TBF so that Yehudi camouflage 
could be factory-installed. So far as is known, 
neither of these projects was completed. 


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CAMOUFLAGE OF NAVY GUIDED MISSILES 


241 


68 CAMOUFLAGE OF NAVY GUIDED 
MISSILES 

During 1944, the Special Designs Section of the 
Navy Bureau of Aeronautics requested the technical 
assistance of Section 16.3 of NDRC in the design of 
Yehudi camouflage for an LBT-1 Glomb. Because 
production of the Glomb had already begun, the 
Section was requested to devise Yehudi gear in the 
form of attachments which might be installed in the 
field, and which would enable this guided missile to 


units, and for their installation on one of the ships 
being constructed by them under their contract with 
the Navy. The electronics staff of the Research Lab¬ 
oratories of the Interchemical Corporation, New 
York, was requested under Contract No. OEMsr- 
697 to design, construct, and install an automatic, 
photoelectric control system which would make man¬ 
ual adjustment of the intensity of the Yehudi lamps 
during flight unnecessary. This equipment was built 
(Figure 24) and field-tested at full scale on the 
premises of the Louis Comfort Tiffany Foundation, 



Figure 24. Automatic photoelectric current control for Yehudi lamps. 

This equipment was built under contract OEMsr-697 for use on LBE-1 Glombs and is described in OSRD Report No. 6556. Two 
photocells (center), on long shielded cables, monitor one of the Yehudi lamps and the sky behind the plane respectively. The photocell 
currents are compared by the vacuum-tube bridge (center), and a relay (round can) controls servo-motor (left) which drives the special 
carbon-pile rheostat. 


approach its target within 6 seconds of flying time 
before becoming visible. Structural and aerodynamic 
difficulties made this problem appear impossible of 
solution. 

On November 1, 1944, the production of LBT-1 
Glombs was discontinued and the Section was asked 
about the feasibility of installing Yehudi camouflage 
on the LBE-1 Glomb then under development by 
Pratt, Read & Company, Inc., Deep River, Connec¬ 
ticut. After a preliminary investigation had dis¬ 
closed that the technical difficulties encountered in 
the LBT-1 were not present in the new LBE-1, the 
Navy requested the Section to supervise the engi¬ 
neering and installation at the factory of Yehudi 
camouflage on an experimental LBE-1. This request 
was formalized by A/N Project Control No. NA- 
188. In response thereto, an OSRD contract 
(OEMsr-1459) was placed with Pratt, Read & Com¬ 
pany Inc. for the mechanical design of the lighting 


Oyster Bay, New York, where an OSRD contract 
(OEMsr-597) for camouflage field studies was in 
force. A description of the control equipment ap¬ 
pears in OSRD Report No. 6556. 8b 

The work by Pratt, Read & Company Inc. was in¬ 
terrupted on several occasions by changing require¬ 
ments imposed by the Navy, including a change in 
the method of intelligence by which the Glomb is 
caused to home on its target. At the time of the 
Japanese surrender, the engineering had been com¬ 
pleted, and a special wing bearing the Yehudi lamps 
nearly constructed. The Navy subsequently canceled 
its contract with Pratt, Read & Company Inc. 
thereby making it impossible for the contractor to 
complete the subject work of Contract No. OEMsr- 
1459. At the request of the Bureau of Ships, the 
apparatus constructed by the Interchemical Corpo¬ 
ration and by Pratt, Read & Company Inc. was 
transferred to the Navy. 


CONFIDENTIAL 



Chapter 7 

ANTISEARCHLIGHT CAMOUFLAGE FOR AIRCRAFT 


71 INTRODUCTION 

D uring 1942, the camouflage officer at Eglin 
Field, Florida, visited the headquarters of Sec¬ 
tion 16.3 of NDRC to discuss the application of 
various camouflage measures to Army aircraft. Dur¬ 
ing the discussion, it was reported that the matte 
black finish used for antisearchlight camouflage was 
not effective, and that its roughness resulted in a 
decrease in the airspeed. 

72 COFFIN PAINT 

The development of an improved type of matte 
black finish was referred to the Research Labora¬ 
tories of the Interchemical Corporation, already 
operating under an OSRD contract (OEMsr-697) 
supervised by this section. In less than two weeks, 
the contractor had produced a novel type of finish 
in which, by the use of a suitable plastic, the small 
particles of carbon black are formed into agglomer¬ 
ates of sufficient size to impart the necessary optical 
roughness to the surface, the function of the plastic 
being somewhat similar to that of the molasses in a 
popcorn ball. Since the plastic is transparent in 
ordinary vehicles, this finish, although smooth by 
ordinary criteria, is extremely matte, and has a 
diffuse reflectance of only 2.2 per cent. Samples of 
this finish were sent to the Army Air Forces Prov¬ 
ing Ground Command at Eglin Field for flight 
tests. 

The results of the Eglin Field tests of this coffin 
paint, as the material came to be called, indicated 
no significant improvement in concealment, the 
observers reporting that aircraft camouflaged with 
this material “looked white in a searchlight beam.” 
They were, in fact, almost indistinguishable from 
planes camouflaged with the standard Army matte 
black finish, which has a diffuse reflectance in the 
neighborhood of 5 per cent. It was concluded from 
these tests that, since a reduction from 5 per cent 
to 2.2 per cent had produced only slight improve¬ 
ment, a much greater reduction would be necessary 
before concealment in a searchlight beam could be 
achieved. This conclusion was borne out by the 


results obtained concurrently at the Tiffany Founda¬ 
tion in a laboratory study on the visibility of tar¬ 
gets. 

The possibility of further reduction in the already 
low diffuse reflectance of coffin paint seemed remote. 
Of the light reflected by coffin paint, nearly all of 
the 2.2 per cent is reflected at the surface of the 
paint without entering the body of the paint film at 
all. Hence, a more complete absorption of light by 
the black pigment itself would not decrease the 
reflectance of the paint film significantly. 

73 GLOSSY BLACK PAINT 

In view of the experience with coffin paint, it 
appeared that the only hope of rendering an air¬ 
plane invisible in searchlight beams lay in the pos¬ 
sibility of altering the geometrical distribution of 
the reflected light. Whereas an aircraft painted with 
coffin paint is almost equally visible from every 
direction when caught in the beam, it seemed that a 
glossy black paint might make an aircraft invisible 
from some directions at the expense of rendering it 
somewhat more visible in others. If the directions 
in which it is visible should happen to be of little 
military importance, the advantage of being invis¬ 
ible under all other circumstances might be very 
great. 

The Interchemical Corporation was asked, there¬ 
fore, to produce a glossy black finish for aircraft 
having the lowest possible diffuse reflectance. This 
meant a dispersion of carbon black in a suitable 
vehicle without extenders or fillers of any kind. The 
material developed is described briefly in Section 
7.5, and its development has been reported in detail 
by the contractor. 1811 

7,3,1 Preliminary Tests at Model Scale 

A sample of the new material was turned over 
to the Tiffany Foundation for field testing. Several 
identical 18-inch models of a B-24 aircraft were 
obtained and painted in the new finish, in coffin 
paint, and with the standard Army camouflage. 
The models were suspended from a steel cable be- 


242 


CONFIDENTIAL 


GLOSSY BLACK PAINT 


243 


tween the 100-foot steel towers shown in Figure 6, 
Chapter 6. Automobile spotlights and battery- 
powered searchlights were used to view the models. 

From the moment of the first comparison, it was 
plainly evident that the new camouflage was vastly 
superior to the matte finish. Indeed, under many 
circumstances, the glossy plane could not be seen 
at all. The appearance of the models is illustrated in 
Figures 1 to 4. These photographs, made as illustra¬ 
tions for this report at a later date, were produced 
with the arrangement of lights, cameras, and models 
shown in Figure 5. 

Figures 1 through 4 show the models as they 
appeared to an observer stationed at or near the 
lights. On the side of the model opposite from the 
lights, there is a point (or points) from which light 
specularly reflected by the Black Widow finish ren¬ 
ders the plane visible, as shown in Figure 6. How¬ 
ever, in the case of a moving airplane, the point 
from which it is visible is moving also, at double 
the ground speed of the plane. An observer, there¬ 
fore, is afforded only a fleeting glimpse of the plane. 
The shortness of the time interval during which the 
plane is visible is important, for it makes the target 
nearly impossible to follow with searchlights or 
guns. 

Flight Tests by the Army 

After the successful experiments at model scale, 
samples of the glossy black finish were sent to Eglin 
Field for flight test. The Army a report which was 
issued subsequently describes the results as follows: 

On most occasions the invisibility of the subject black 
camouflage with searchlights full on the airplane is amazing. 
The standard daytime camouflage is visible as a silvery air¬ 
plane during the entire traverse across the searchlights. The 
standard dull black camouflage is almost always as visible, 
but does not shine as brightly. The subject black camouflage 
is invisible most of the time. 

Optically controlled searchlights were said to be 
quite unable to find and hold the airplane at all, 
and the effectiveness of radar-controlled search¬ 
lights was reduced about 50 per cent. Even when the 
test plane was held in the beam of a radar-controlled 
searchlight, 8-power night binoculars enabled only 
the insignia and the revolving propellers to be dis¬ 
tinguished. These results were not wholly unex- 

a Final Report on Test of Glossy Paint for Night Camou¬ 
flage, Serial No. 3-43-114, AAF Bd. Project No. (M-l) 17, 
Proof Department, Army Air Forces Proving Ground Com¬ 
mand, Eglin Field, Florida. 


pected, in view of the, fact that this finish has a 
diffuse reflectance in the neighborhood of 0.1 per 
cent. 

The Procurement Problem 

In view of the favorable report from the Air 
Forces Proving Ground, it seemed likely that more 
extensive tests and demonstrations would be con¬ 
ducted, and that such tests would require more of 
the glossy black finish than the Research Labora¬ 
tories of the Interchemical Corporation would be 
able to produce with their small-scale equipment. 
Arrangements were made, therefore, with the Ault 
& Wiborg Corporation, Cincinnati, the Mallinckrodt 
Chemical Works, St. Louis, and the Rohm & Haas 
Company, Philadelphia, to compound materials 
identical in formulation with the material used in 
the Eglin Field tests. These materials were made 
available under the trade names of Wiblack, 
Mallo Black, and Rhoco Black respectively. By 
establishing three sources of supply, the material 
could be procured by the Army or Navy without 
waiting for a specification to be prepared. 

Adoption by the Army 

The contemplated tests were never conducted be¬ 
cause the Chief of the Army Air Forces ordered 
that all night fighters be provided with this type of 
camouflage. The Section immediately informed the 
Army Air Forces Materiel Command at Wright 
Field concerning all pertinent details of this devel¬ 
opment, and the regular Army procurement proce¬ 
dures were instituted. 

At this time, the code name, Black Widow Proj¬ 
ect, was adopted in recognition of the P-61 night 
fighter, which was the first type of aircraft to be 
given this antisearchlight protection. 

Inquiries from Britain 

The London Mission of the OSRD had been kept 
informed of the progress of this development, and, 
as a result, cables from Britain began to request 
more information concerning the antisearchlight 
finish, primarily in connection with night-bombing 
operations based in England. Inasmuch as the nec¬ 
essary information could not be readily embodied 
in correspondence or reports, the Section suggested 
to the War Department that the Assistant Chief, 
Miscellaneous Section, Proving Ground Command, 


CONFIDENTIAL 




244 


ANTISEARCHLIGHT CAMOLFLAGE FOR AIRCRAFT 


Figures 1 through 4. Photographs of models of B-24 aircraft as seen against the night sky when fully illuminated 
by 3 searchlights located near the camera. The model on the left is painted matte black; the model on the right 
has the Black Widow' finish. 



Figure 1 



Figure 2 


CONFIDENTIAL 




GLOSSY BLACK PAINT 


245 



Figure 3 



Figure 4 


CONFIDENTIAL 







246 


ANTISEARCHLIGHT CAMOUFLAGE FOR AIRCRAFT 



Figure 5. Arrangement of camera, lights, and model 
aircraft used to secure photographs shown in Figures 
1.2. 3. 4. and 7. 


be sent to England on a special mission to acquaint 
both the AAF and the RAF with this new camou¬ 
flage measure. 

This was done, and during the four months he 
was there, the application of this finish on a large 
number of aircraft was supervised, a routine sched¬ 
ule for refinishing was established, and arrange¬ 
ments for suitable production facilities in the United 
Kingdom were made. 

Use over Germany 

Memorandum Report No. 283 from the Air Tech¬ 
nical Section, Headquarters, European Theater of 
Operations, subject, Application and Observation 
of Antisearchlight Camouflage in E.T.O., contains 
the first indication of the effectiveness of this anti¬ 
searchlight camouflage under combat conditions. 
Although only relatively few bombers had been re¬ 
finished at the time that this report was issued, 
there had been enough instances of effective protec¬ 
tion against enemy searchlights to enable the report 
to take cognizance of the improvement in the morale 
of squadrons using Black Widow finish. 



Figure 6. Photograph of models of B-24 aircraft^ showing light reflected specularly by Black Widow finish (model 
on right) when the plane is between the observer and tha searchlights. In the case of a moving plane, any observer 
■zees this condition only for an instant. 


CONFIDENTIAL 









FORMULATION OF THE BLACK WIDOW FINISH 


247 


Inquiries from the Pacific 

U. S. Air Forces stationed in the Pacific theaters, 
on receipt of the report from Eglin Field mentioned 
above, requested technical assistance in connection 
with this new antisearchlight camouflage. A con¬ 
siderable quantity of this paint, procured under 
specifications prepared by Wright Field, was shipped 
to the various theaters, and the camouflage officer 
was sent to the central and south Pacific areas 
shortly after his return from England. He was as¬ 
sisted by civilian members of the Office of Field 
Service. In this way, all Air Forces involved in the 
war with Japan were apprised of the value of this 
camouflage measure and were given assistance in 
the application and maintenance of the finish. 

Because the operational conditions in the Pacific 
theater are so unlike those that exist in Europe, 
the effectiveness of this antisearchlight measure was 
subjected anew to flight tests over searchlight bat¬ 
teries manned by veteran crews in one of the active 
theaters. These tests brought out advantages which 
had not been realized previously, and suggested new 
tactics which had not been employed formerly. The 
results of these tests are to be found in Memoran¬ 
dum Report on Antisearchlight Camouflage , Oper¬ 
ations Analysis Section, Headquarters, Far Eastern 
Air Forces, the classification of which is higher than 
that of this report. 

7 4 OPTICAL BEHAVIOR OF THE BLACK 
WIDOW FINISH 

An aircraft camouflaged with Black Widow finish 
has a diffuse reflectance of approximately 0.1 per 
cent, so that in a searchlight beam it appears about 
1/1000 as bright as would a white-painted ship. To 
this diffusely reflected light is added the contribution 
due to all of the tiny virtual images of the search¬ 
lights formed by the mirror-like surface of the paint. 

If aircraft were geometrically simple shapes, such 
as planes, cylinders, or spheres, it would be an easy 
matter to calculate the size of the virtual images of 
the searchlights. Since most aircraft surfaces are 
convex, the virtual images are generally small, lo¬ 
cated within the ship, and visible over a wide range 
of directions. The brightness of each image is ap¬ 
proximately 4 per cent of the brightness of the 
searchlight itself, neglecting the attenuation due to 
atmospheric haze between the light and the plane. 

When the plane is considered as a visual target, 


the inherent integrated contrast of the plane will be 
zero if the total light reflected toward the observer 
equals the total light which the observer would have 
received from the part of the sky which the plane 
obstructs. Because the space behind the plane is 
lighted by the searchlight beam, the obstructed light 
may exceed the amount received normally from the 
night sky. Therefore, zero reflected light from the 
plane is not the condition for minimum visibility. 

No opportunity was found during the war to 
measure the quantities of light involved in an actual 
service test. However, the reports from the Services 
indicate that the Black Widow finish affords more 
perfect concealment for the planes caught in search¬ 
light beams than was originally expected. This suc¬ 
cess may be explained if, by chance, the condition 
for zero inherent integrated contrast is met by a 
plane of conventional shape treated with Black 
Widow finish. 

7 5 FORMULATION OF THE BLACK WIDOW 
FINISH 

Black Widow finish may be prepared and used in 
a variety of forms, such as lacquer, fast-drying 
enamel and enamel of medium drying rate. The 
type of finish which should be chosen for use in any 
one location depends upon such factors as the shel¬ 
ter afforded, the time available before the aircraft 
must be returned to service, the atmospheric con¬ 
ditions, and the finishing equipment installed. 

The drying of lacquer depends almost entirely 
on the evaporation of the solvents in the lacquer, 
while the drying of enamel is related not only to 
the evaporation of the solvents in it but also, as in 
ordinary paints, to the oxidation of some of the 
resinous constituents in the binder, which converts 
them into tough, insoluble films. Lacquers dry dust- 
free much more rapidly than enamels and therefore 
do not require as much shelter during application 
as enamels. In general, the desired coverage may be 
obtained in fewer coats with enamel than with 
lacquer, which contains less solids. Also, enamel 
yields a somewhat higher gloss than lacquer, be¬ 
cause the more rapid evaporation of solvents, char¬ 
acteristic of the lacquer form, tends to impart to 
the surface a mild unevenness called orange-peel. 
Except in aggravated or neglected cases, however, 
this orange-peel does not detract from the perform¬ 
ance of Black Widow lacquer as a camouflage finish 
compared to Black Widow enamel. 


CONFIDENTIAL 




248 


ANTISEARCHLIGHT CAMOUFLAGE FOR AIRCRAFT 


In preliminary discussion with the Army Air 
Forces Proving Ground Command, Eglin Field, 
Florida, a preference was expressed for the syn¬ 
thetic enamel type of formulation, which promised 
to be the easier material to apply to aircraft pre¬ 
viously finished with Air Corps camouflage mate¬ 
rials. For this reason the enamel type of Black 
Widow finish was the first supplied. Subsequent 
events suggested that Black Widow finish might not 
be applied to any considerable extent in modifica¬ 
tion centers as a repaint job, but would find its 
greatest use in production as the original finish, 
where rapid drying is of paramount importance to 
permit further work on the plane as soon after 
painting as possible. When Black Widow finish is 
shipped for application elsewhere, the greater stor¬ 
age stability of lacquers as compared with enamels, 
particularly for Pacific areas with their considerable 
transit times, further recommended the lacquer 
type. In addition, since painting facilities are often 
crude and time is of vital importance in advanced 
theaters, the lacquer type will be in much greater 
demand. Both forms, however, receive attention 
here. 

Like all coating compositions, Black Widow fin¬ 
ish consists of pigment, vehicle, and solvent. Carbon 
black alone seems permissible as the pigment, and 
the desired results, will be obtained only from the 
best lacquer grades of carbon black, selling in the 
neighborhood of 50 cents and more per pound. The 
gas blacks generally used for compounding rubber, 
in news ink, and in paints are not suitable. Further, 
the carbon black chosen must be very well dis¬ 
persed by one of several highly efficient methods 
known to the art, for any clumps of undispersed 
black will mar the final finish and disturb the de¬ 
sired low-diffuse reflectance. 

For Black Widow lacquer, the vehicle generally 
consists of a mixture of suitable grades of nitro¬ 
cellulose and one or more synthetic resins. In the 
case of Black Widow enamel, the binder in the 
vehicle is made up of one or more resins, chosen to 
give in combination the desired ultimate properties 
in the film. To accelerate the air-drying of the 
enamel coat, conventional metallic driers are 
added. 

Sufficient solvents and diluents are present or 
added to the coating compositions to convert them 
to the proper consistency for the selected method 
of application. 


7,5-1 Typical Formulations 

The composition of typical formulations is as 
follows: 

Typical Black Widow Lacquer 


Ingredients 

Parts by Weight 

Carbon black (high color) 

5.0 


Nitrocellulose (low viscosity) 

40.0 


Alkyd resin (nonoxidizing type) 

45.0 


Plasticizer 

10.0 


Solids 


100.0 

Volatile lacquer solvent and diluent 


225.0 

Total 


325.0 


For spraying, thin 3 parts above with 2 parts of lacquer 
thinner. 


Typical Black Widow Enamel 


Ingredients 

Parts by Weight 

Carbon black (high color) 

6.5 


Alkyd resin (oxidizing type) 

84.5 


Mixed drier solution 

9.0 


Solids 


100.0 

Volatile solvent 


90.0 

Total 


190.0 


For spraying, thin 2 parts above with 1 part of enamel 
thinner. 

76 APPLICATION OF BLACK WIDOW 
FINISH 

In the application of either the lacquer or the 
enamel type of Black Widow finish, standard indus¬ 
trial painting procedure is followed. No special tech¬ 
niques are required. The lacquer or enamel, as the 
case may be, is thinned to the proper consistency 
for spraying and applied by means of the spraying 
facilities with which Air Corps service installations 
are equipped. 

When a repaint job over other camouflage is nec¬ 
essary, it is not possible to avoid the sanding of the 
previous paint since conventional camouflage fin¬ 
ishes are comparatively very rough and porous and 
are not adapted to receive a glossy top coat. The 
old surface is first sanded smooth with No. 320 or 
No. 400 abrasive paper and water. The scum which 
is left after the water sanding is carefully wiped 
away. The Black Widow finish is then applied by 
standard spraying methods. 

On jobs old or new, Black Widow finish need not 
be applied to the entire fuselage. The gloss black is 
sprayed on all under surfaces and carried three- 
quarters of the way up the sides. When viewed from 
below or from any low lateral position, only un- 


CONFIDENTIAL 







MAINTENANCE OF THE BLACK WIDOW FINISH 


249 


broken glossy black surfaces should present them¬ 
selves. 

The top insigne is left undisturbed. The identifi¬ 
cation insignia on the bottom are completely cov¬ 
ered. The side insigne is usually dulled by fogging 
with gray paint or by a light pass of the Black 
Widow finish. 

Detailed instructions for the application of this 
paint are contained in Headquarters Army Air 
Forces Technical Order 0-7-1-1, which also gives 
some information as to coverage and procurement. 


Like all lacquers and enamels, Black Widow fin¬ 
ish gradually shows the normal cumulative effects of 
weathering. After the equivalent of a two-week 
exposure in Florida, the spectral reflectance of the 
cleaned surface is found to increase to a value of 
about 0.5 per cent (after the gentle removal of the 
scum collected in this period). Conditions on an air¬ 
field and in flight are more rigorous than standard 
Florida exposure, for the clouds of abrasive dust 
stirred up by propellers on the ground and caked in 
wet weather on the aircraft surfaces, and the cor- 



Figure 7. Photograph identical with Figure 3 except that the Black Widow finish on the model on the right has 
been splashed with mud, thus giving parts of the plane a high diffuse reflectance. This illustrates the importance of 
keeping the Black Widow finish clean and glossy. Maintenance procedures are discussed in Section 7.7. 


MAINTENANCE OF THE BLACK 
WIDOW FINISH 

The most important attribute of Black Widow 
finish is, of course, its exceptionally low diffuse re¬ 
flectance (Figure 7). With reasonable care in the 
selection of raw materials, as outlined in the pre¬ 
ceding section, the diffuse reflectance is readily kept 
at a value of 0.15 per cent or lower. Since informa¬ 
tion concerning methods of maintaining Black 
Widow finish does not appear in practicable form 
elsewhere, it is included in the following paragraphs. 


rosive gases eliminated in proximity to gun mount¬ 
ings, hasten the destruction of the desirable low T 
reflectance. It was therefore considered essential 
that methods for the maintenance of the low re¬ 
flectance of Black Widow finish be investigated. 

Based upon a long series of experiments with 
various polishes, waxes, and rejuvenating coats, a 
set of maintenance recommendations was formu¬ 
lated for maintenance installations offering moder¬ 
ately complete facilities and ample time for the nec¬ 
essary operations: 

1. On a fresh surface, whether of the enamel or 


CONFIDENTIAL 




250 


ANTISEARCHLIGHT CAMOUFLAGE FOR AIRCRAFT 


lacquer type, all additional treatment results only 
in a finish of increased reflectance and should there¬ 
fore be avoided. 

2. For a fresh finish which has become dusty or 

is covered only superficially with dirt, the best pro¬ 
cedure for restoring the original low reflectance 
consists of washing away the foreign material with 
a mild detergent or soap containing a minimum of 
alkali. % 

3. A mildly weathered surface may be brought 
back to a satisfactory low level of reflectance with 
an extremely mild abrasive. In the case of a lacquer 
finish, the reflectance may be still further reduced 
by application thereafter of a waxy compound with 
a soft cloth, while care is taken not to leave any 
whitened areas of excess wax. Enamel finishes, how¬ 
ever, are in general softer than typical lacquers, 
which makes waxing of the enamel undesirable be¬ 
cause it introduces fresh scratches. 

4. When a finish has become badly abraded but 
still consists of a continuous unbroken film, whether 
of lacquer or of enamel, it is first cleaned of all dirt, 
dust and other foreign materials by washing with a 
mild soap and water or by cleaning with a very 
mild abrasive, and the entire surface is then re¬ 
juvenated by application of a very thin coat of 
enamel. This new coat may be applied with a cloth 
soaked with diluted enamel. A diluted lacquer com¬ 
position is not suitable for this purpose because, 
when used, it will dissolve the lacquer coat pre¬ 
viously applied to give a finish of considerably in¬ 
creased reflectance. On enamel, it is much more 
difficult to apply than a rejuvenating enamel finish. 
A second, additional rejuvenation coat may be 
applied after weathering of the rejuvenated finish. 

5. If the lacquer or enamel finish is broken, 
burned, or so badly marred that it gives unsatis¬ 
factory results when treated according to the proce¬ 
dures outlined above, another full finishing coat, 
comparable in thickness to those which preceded it, 
is required. To reproduce the original reflectance 
characteristics, the surface to be recoated should 
be smooth, which often necessitates preliminary 


sanding followed by washing. The use of lacquer 
over enamel or enamel over lacquer for this addi¬ 
tional full coat is to be avoided, as the resultant film 
will show poor adhesion to and (in the case of 
lacquer over enamel) will lift the coats below. 

In combat theaters, however, the necessary equip¬ 
ment and time for the execution of the recommenda¬ 
tions above are not always available. In these in¬ 
stances, emergency measures must be adopted for 
the rejuvenation of the surfaces w T hich are encrusted 
with mud or marred by muzzle blast and engine ex¬ 
haust. Under these conditions, the aircraft is first 
washed thoroughly with water from a steam jenny. 
The temperature and pressure of the water used are 
adjusted to cope with the mud crusts. Soap may be 
added to the water to assist in its detergent action. 
Any dull spots which remain after the washing has 
been completed are then polished with wax free of 
abrasive. If wax polish is not available, the dull 
spot may be wiped with lubricating oil or hydraulic 
fluid. Such treatments may not, of course, reduce the 
nonspecular reflectance to the desired low value but 
have been found in practice to serve adequately 
under the circumstances. 

78 OPERATIONAL RESULTS 

Numerous informal reports of the successful oper¬ 
ational use of Black Widow camouflage reached 
Section headquarters. In the opinion of the Section, 
the evolution of this camouflage measure was its 
most important contribution to the war effort. If 
commonly quoted figures regarding the cost of a 
bomber and its crew are multiplied by the number 
of bombers which appear to have been saved, the 
total expenditures by the Camouflage Section are 
dwarfed to the point of insignificance. After taking 
account of the saving in the lives of bomber crews 
and of the possible increase in bombing efficiency 
which resulted from the use of the Black Widow 
finish, Section 16.3 of NDRC has come to feel a 
deep sense of pride in the development of this cam¬ 
ouflage measure. 


CONFIDENTIAL 



APPENDIX A 


Values of the liminal contrast of circular targets as read from a large-scale plot of Figure 35, Chapter 3. The various 
values of the angular subtense of the target were chosen to facilitate the preparation of the nomographic visibility charts. 


Angular 

subtense 


L I 


M I N A L 


CONTRAST 


(FOOT-LAMBERTS) 


of target 
(minutes) 

1,000 

100 

10 

1 

10 _1 

10~ 2 

10- 3 

10- 4 

10" 5 

358.9 

0.00272 

0.00272 

0.00277 

0.00334 

0.00534 


0.0303 

0.0624 

0.136 

340.4 

0.00272 

0.00272 

0.00277 

0.00334 

0.00536 

0.0112 

0.0308 

0.0637 

0.140 

340.0 

0.00272 

0.00272 

0.00277 

0.00334 

0.00537 

0.0112 

0.0308 

0.0638 

0.140 

323.0 

0.00272 

0.00272 

0.00277 

0.00335 

0.00539 

0.0114 

0.0314 

0.0652 

0.144 

302.6 

0.00272 

0.00272 

0.00277 

0.00335 

0.00542 

0.0116 

0.0320 

0.0664 

0.147 

293.6 

0.00272 

0.00272 

0.00277 

0.00335 

0.00544 

0.0117 

0.0325 

0.0678 

0.151 

291.8 

0.00272 

0.00272 

0.00277 

0.00335 

0.00544 

0.0117 

0.0326 

0.0679 

0.152 

280.9 

0.00272 

0.00272 

0.00278 

0.00335 

0.00547 

0.0119 

0.0330 

0.0690 

0.155 

269.2 

0.00272 

0.00272 

0.00278 

0.00335 

0.00550 

0.0120 

0.0335 

0.0703 

0.159 

258.4 

0.00272 

0.00272 

0.00278 

0.00335 

0.00553 

0.0121 

0.0340 

0.0716 

0.164 

255.3 

0.00272 

0.00272 

0.00278 

0.00335 

0.00553 

0.0122 

0.0341 

0.0720 

0.164 

234.9 

0.00272 

0.00272 

0.00278 

0.00336 

0.00558 

0.0124 

0.0352 

0.0748 

0.172 

226.9 

0.00272 

0.00272 

0.00278 

0.00336 

0.00562 

0.0126 

0.0356 

0.0760 

0.176 

215.3 

0.00272 

0.00272 

0.00279 

0.00336 

0.00565 

0.0128 

0.0364 

0.0780 

0.182 

204.3 

0.00272 

0.00272 

0.00279 

0.00336 

0.00569 

0.0129 

0.0370 

0.0800 

0.188 

198.8 

0.00272 

0.00272 

0.00279 

0.00337 

0.00570 

0.0130 

0.0376 

0.0811 

0.191 

185.7 

0.00272 

0.00272 

0.00279 

0.00338 

0.00575 

0.0133 

0.0386 

0.0840 

0.200 

184.6 

0.00272 

0.00272 

0.00279 

0.00338 

0.00577 

0.0133 

0.0386 

0.0842 

0.201 

172.3 

0.00273 

0.00273 

0.00279 

0.00339 

0.00581 

0.0136 

0.0398 

0.0875 

0.210 

170.2 

0.00273 

0.00273 

0.00279 

0.00339 

0.00582 

0.0136 

0.0401 

0.0880 

0.212 

161.5 

0.00273 

0.00273 

0.00279 

0.00340 

0.00588 

0.0138 

0.0410 

0.0907 

0.220 

157.1 

0.00273 

0.00273 

0.00279 

0.00340 

0.00589 

0.0140 

0.0415 

0.0922 

0.224 

152.0 

0.00274 

0.00274 

0.00279 

0.00340 

0.00593 

0.0141 

0.0422 

0.0940 

0.230 

145.9 

0.00274 

0.00274 

0.00279 

0.00341 

0.00596 

0.0143 

0.0430 

0.0963 

0.237 

143.6 

0.00274 

0.00274 

0.00279 

0.00341 

0.00597 

0.0144 

0.0434 

0.0973 

0.240 

136.2 

0.00274 

0.00274 

0.00279 

0.00342 

0.00603 

0.0146 

0.0446 

0.101 

0.250 

136.0 

0.00274 

0.00274 

0.00280 

0.00342 

0.00603 

0.0146 

0.0446 

0.101 

0.250 

129.2 

0.00275 

0.00275 

0.00280 

0.00343 

0.00608 

0.0149 

0.0459 

0.104 

0.259 

127.7 

0.00275 

0.00275 

0.00280 

0.00343 

0.00608 

0.0150 

0.0461 

0.104 

0.263 

120.1 

0.00275 

0.00275 

0.00280 

0.00344 

0.00615 

0.0153 

0.0476 

0.109 

0.274 

117.5 

0.00276 

0.00276 

0.00280 

0.00345 

0.00617 

0.0154 

0.0482 

0.110 

0.280 

113.5 

0.00276 

0.00276 

0.00280 

0.00345 

0.00621 

0.0156 

0.0493 

0.113 

0.287 

107.7 

0.00276 

0.00276 

0.00281 

0.00347 

0.00627 

0.0159 

0.0508 

0.118 

0.301 

107.5 

0.00277 

0.00277 

0.00281 

0.00347 

0.00627 

0.0160 

0.0508 

0.118 

0.301 

102.1 

0.00277 

0.00277 

0.00281 

0.00348 

0.00634 

0.0163 

0.0523 

0.122 

0.315 

99.38 

0.00277 

0.00277 

0.00281 

0.00349 

0.00638 

0.0165 

0.0536 

0.125 

0.323 

97.26 

0.00277 

0.00277 

0.00281 

0.00349 

0.00639 

0.0166 

0.0540 

0.127 

0.328 

92.84 

0.00278 

0.00278 

0.00282 

0.00351 

0.00646 

0.0169 

0.0554 

0.131 

0.343 

92.29 

0.00278 

0.00278 

0.00282 

0.00351 

0.00646 

0.0169 

0.0562 

0.132 

0.344 

88.80 

0.00278 

0.00278 

0.00282 

0.00352 

0.00652 

0.0172 

0.0572 

0.136 

0.356 

86.13 

0.00278 

0.00278 

0.00283 

0.00352 

0.00656 

0.0175 

0.0581 

0.139 

0.366 

85.10 

0.00278 

0.00278 

0.00283 

0.00352 

0.00659 

0.0176 

0.0586 

0.140 

0.371 

81.70 

0.00279 

0.00279 

0.00283 

0.00353 

0.00664 

0.0179 

0.0605 

0.145 

0.386 

80.75 

0.00279 

0.00279 

0.00284 

0.00355 

0.00667 

0.0180 

0.0607 

0.146 

0.389 

76.00 

0.00279 

0.00279 

0.00284 

0.00358 

0.00675 

0.0184 

0.0632 

0.154 

0.413 

74.28 

0.00279 

0.00279 

0.00284 

0.00358 

0.00679 

0.0187 

0.0643 

0.157 

0.422 

71.78 

0.00280 

0.00280 

0.00285 

0.00360 

0.00685 

0.0190 

0.0658 

0.162 

0.436 

68.08 

0.00280 

0.00280 

0.00286 

0.00361 

0.00695 

0.0194 

0.0684 

0.169 

0.462 

68.00 

0.00280 

0.00280 

0.00286 

0.00361 

0.00696 

0.0195 

0.0686 

0.170 

0.462 

64.60 

0.00281 

0.00281 

0.00286 

0.00365 

0.00705 

0.0200 

0.0710 

0.177 

0.485 

62.85 

0.00281 

0.00281 

0.00287 

0.00366 

0.00710 

0.0202 

0.0725 

0.182 

0.501 

58.73 

0.00282 

0.00282 

0.00289 

0.00369 

0.00724 

0.0209 

0.0764 

0.194 

0.537 

58.36 

0.00282 

0.00282 

0.00289 

0.00369 

0.00725 

0.0210 

0.0767 

0.194 

0.541 

54.47 

0.00284 

0.00284 

0.00290 

0.00374 

0.00741 

0.0218 

0.0809 

0.208 

0.583 


CONFIDENTIAL 


251 


































252 


APPENDIX A 


Angular 

subtense 

L 

I M I N 

A L C 

ONTRAST (FOOT 

-LAMBERTS) 

of target 
(minutes) 

1,000 

100 

10 

1 

10-’ 

10" 2 

10“ 3 

10" 4 

10' 5 

53.83 

0.00284 

0.00284 

0.00290 

0.00374 

0.00743 

0.0220 

0.0818 

0.210 

0.5)1 

51.06 

0.00285 

0.00285 

0.00292 

0.00378 

0.00756 

0.0225 

0.0850 

0.222 

0.627 

49.69 

0.00286 

0.00286 

0.00293 

0.00380 

0.00763 1 

0 0229 

0.0874 

0.228 

0.649 

48.06 

0.00286 

0.00286 

0.00294 

0.00382 

0.00771 

0.0233 

0.0897 

0.236 

0.673 

46.14 

0.00287 

0.00287 

0.00295 

0.00385 

0.00782 

0.0238 

0.0926 

0.246 

0.708 

45.39 

0.00288 

0.00288 

0.00296 

0.00386 

0.00785 

0.0240 

0.0940 

0.250 

0.721 

43.07 

0.00290 

0.00290 

0.00298 

0.00390 

0.00802 

0.0248 

0.0982 

0.265 

0.767 

43.00 

0.00290 

0.00290 

0.00298 

0.00390 

0.00802 

0.0249 

0.0984 

0.265 

0 768 

40.85 

0.00292 

0.00292 

0.0C301 

0.00394 

0.00815 

0.0256 

0.103 

0.280 

0.818 

40.38 

0.00292 

0.00292 

0.00301 

0.00395 

0.00820 

0.0258 

0.104 

0.283 

0.831 

38.00 

0.00294 

0.00294 

0.00304 

0.00402 

0.00840 

0.0267 

0.110 

0.303 

0.896 

37.14 

0.00295 

0.00295 

0.00305 

0.00404 

0.00845 

0.0271 

0.112 

0.312 

0.925 

36.91 

0.00295 

0.00295 

0.00306 

0.00405 

0.00848 

0.0272 

0.113 

0.314 

0.930 

35.89 

0.00296 

0.00296 

0.00307 

0.00407 

0.00857 

0.0277 

0.116 

0.324 

0.967 

34.04 

0.00299 

0.00299 

0.00310 

0.00413 

0.00876 

0.0286 

0.122 

0.344 

1.04 

34.00 

0.00299 

0.00299 

0.00310 

0.00413 

0.00881 

0.0287 

0.122 

0.345 

1.04 

32.30 

0.00302 

0.00302 

0.00313 

0.00420 

0.00895 

0.0297 

0.128 

0.367 

1.12 

31.42 

0.00304 

0.00304 

0.00314 

0.00422 

0.00904 1 

0.0302 

0.131 

0.380 

1.16 

30.76 

0.00305 

0.00305 

0.00316 

0.00425 

0.00913 1 

0.0306 

0.134 

0.389 

1.20 

29.36 

0.00307 

0.00307 

0.00320 

0.00432 

0.00933 

0.0316 

0.141 

0.412 

1.28 

29.18 

0.00308 

0.00308 

0.00321 

0.00432 

0.00934 

0.0317 

0.142 

0.416 

1.30 

28.71 

0.00309 

0.00309 

0.00321 

0.00434 

0.00942 

0.0321 

0.144 

0.425 

1.33 

28.09 

0.00310 

0.00310 

0.00323 

0.00438 

0.00954 

0.0326 

0.148 

0.436 

1.37 

27.23 

0.00312 

0.00312 

0.00327 

0.00442 

0.00966 

0.0332 

0.153 

0.454 

1.44 

26.92 

0.00313 

0.00313 

0.00327 

0.00444 

0.00970 

0.0335 

0.154 

0.460 

1.46 

25.84 

0.00316 

0.00316 

0.00330 

0.00452 

0.00991 

0.0346 

0.161 

0.486 

1.56 

25.53 

0.00316 

0.00316 

0.00331 

0.00453 

0.00994 

0.0348 

0.163 

0.494 

1.58 

24.03 

0.00321 

0.00321 

0.00337 

0.00462 

0.0103 

0.0364 

0.175 

0.537 

1.74 

23.49 

0.00323 

0.00323 

0.00340 

0.00469 

0.0104 

0.0371 

0.179 

0.555 

1.80 

22.69 

0.00326 

0.00326 

0.00344 

0.00474 

0.0106 

0.0381 

0.186 

0.581 

1.91 

21.53 

0.00330 

0.00330 

0.00350 

0.00485 

0.0110 

0.0397 

0.198 

0.625 

2.07 

21.50 

0.00330 

0.00330 

0.00350 

0.00486 

0.0110 

0.0398 

0.199 

0.628 

2.09 

20.43 

0.00335 

0.00335 

0.00357 

0.00498 

0.0113 

0.0414 

0.211 

0.676 

2.27 

19.88 

0.00337 

0.00337 

0.00361 

0.00506 

0.0115 

0.0423 

0.218 

0.703 

2.38 

18.57 

0.00344 

0.00344 

0.00371 

0.00524 

0.0120 

0.0449 

0 237 

0.781 

2.68 

18.46 

0.00345 

0.00345 

0.00371 

0.00526 

0.0120 

0.0452 

0.239 

0.787 

2.71 

17.23 

0.00352 

0.00352 

0.00383 

0.00547 

0.0126 

0.0479 

0.262 

0.877 

3.08 

17.02 

0.00354 

0.00354 

0.00386 

0.00551 

0.0127 

0.0485 

0.266 

0.891 

3.15 

16.15 

0.00360 

0.00360 

0.00395 

0.00569 

0.0132 

0.0508 

0.286 

0.972 

3.44 

15.71 

0.00364 

0.00364 

0.00401 

0.00581 

0.0135 

0 0522 

0.297 

1.02 

3.64 

15.20 

0.00368 

0.00368 

0.00409 

0.00593 

0.0138 

0.0540 

0.312 

1.08 

3.89 

14.59 

0.00374 

0.00370 

0.00417 

0.00611 

0.0143 

0.0562 

0.330 

1.15 

4.21 

14.36 

0.00376 

0.00372 

0.00420 

0.00618 

0.0144 

0.0571 

0.337 

1.19 

4.34 

13.62 

0.00384 

0.00382 

0.00434 

0.00643 

0.0151 

0.0604 

0.365 

1.30 

4.83 

13.60 

0.00384 

0.00382 

0.00436 

0.00644 

0.0152 

0.0605 

0.366 

1.30 

4.84 

12.92 

0.00392 

0.00391 

0.00449 

0.00668 

0.0158 

0.0639 

0.393 

1.43 

5.36 

12.77 

0.00394 

0.00394 

0.00453 

0.00678 

0.0160 

0.0649 

0.401 

1.46 

5.47 

12.01 

0.00406 

0.00407 

0.00473 

0.00713 

0.0170 

0.0695 

0.439 

1.64 

6.18 

11.75 

0.00410 

0.00412 

0.00481 

0.00728 

0.0172 

0.0713 

0.455 

1.71 

6.47 

11.67 

0.00411 

0.00413 

0.00484 

0.00733 

0.0174 

0.0719 

0.460 

1.73 

6 52 

11.35 

0.00417 

0.00419 

0.00493 

0.00750 

0.0179 

0.0742 

0.480 

1.82 

6.93 

10.77 

0.00430 

0.00434 

0.00518 

0.00791 

0.0189 

0.0794 

0.522 

2.03 

7.73 

10.75 

0.00430 

0.00436 

0.00520 

0.00792 

0.0189 

0.0796 

0.524 

2.03 

7 74 

10.21 

0.00443 

0.00450 

0.00542 

0.00836 

0.0200 

0.0847 

0.569 

2.24 

8.55 

9.938 

0.00451 

0.00460 

0.00558 

0.00861 

0.0206 

0.0879 

0.593 

2.37 

9.01 

9.726 

0.00456 

0.00468 

0.00572 

0.00883 

0.0212 

0.0904 

0.616 

2.47 

9.45 

9.284 

0.00470 

0.00485 

0.00598 

0.00931 

0.0224 

0.0965 

0.667 

2.71 

10.3 

9.229 

0.00472 

0.00489 

0.00603 

0.00940 

0.0226 

0.0966 

0.674 

2.74 

10.5 

9.078 

0.00478 

0.00494 

0.00612 

0.00957 

0.0231 

0.0988 

0.692 

2.82 

1 10.8 


CONFIDENTIAL 





































APPENDIX A 


253 


Angular 


LIMINAL CONTRAST (FOOT-LAMBERTS) 


subtense 


of target 
(minutes) 

1,000 

100 

10 

1 

10' 1 

10“ 2 

10- 3 

10~ 4 

10~ 5 

8.880 

0.00485 

0.00506 

0.00629 

0.00984 

0.0237 

0.102 

0.720 

2.95 

11.4 

8.613 

0.00496 

0.00519 

0.00649 

0.0103 

0.0248 

0.107 

0.758 

3.13 

12.0 

8.510 

0.00500 

0.00525 

0.00659 

0.0104 

0.0251 

0.108 

0.774 

3.21 

12.3 

8.170 

0.00518 

0.00544 

0.00696 

0.0110 

0.0266 

0.116 

0.838 

3.49 

13.4 

8.075 

0.00522 

0.00552 

0.00703 

0.0112 

0.0272 

0.117 

0.852 

3.55 

13.6 

7.600 

0.00550 

0.00589 

0.00763 

0.0122 

0.0298 

0.129 

0.956 

4.01 

15.5 

7.430 

0.00562 

0.00605 

0.00787 

0.0126 

0.0309 

0.133 

0.995 

4.20 

16.1 

7.178 

0.00579 

0.00627 

0.00824 

0.0133 

0.0327 

0.140 

1.06 

4.49 

17 3 

6.808 

0.00611 

0.00673 

0.00891 

0.0145 

0.0358 

0.153 

1.19 

5.00 

19.2 

6.800 

0.00611 

0.00675 

0.00892 

0.0146 

0.0359 

0.154 

1.19 

5.01 

19.3 

6.460 

0.00646 

0.00720 

0.00962 

0.0158 

0.0393 

0.167 

1.31 

5.55 

21.4 

6.290 

0.00667 

0.00745 

0.0100 

0.0166 

0.0413 

0.175 

1.38 

5.82 

22.6 

5.873 

0.00721 

0.00824 

0.0113 

0.0188 

0.0468 

0.197 

1.57 

6.68 

25.9 

5.836 

0.00728 

0.00828 

0.0113 

0.0190 

0.0472 

0.199 

1.60 

6.76 

26.2 

5.447 

0.00794 

0.00923 

0.0127 

0.0216 

0.0534 

0.226 

1.83 

7.78 

30.0 

5.383 

0.00807 

0.00943 

0.0130 

0.0220 

0.0546 

0.230 

1.88 

7.97 

30.7 

5.106 

0.00869 

0.0102 

0.0143 

0.0243 

0.0603 

0.254 

2.07 

8.83 

34.2 

4.969 

0.00906 

0.0107 

0.0149 

0.0256 

0.0639 

0.268 

2.19 

9.35 

36.1 

4.806 

0.00955 

0.0114 

0.0159 

0.0275 

0.0681 

0.286 

2.34 

9.98 

38.6 

4.614 

0.0101 

0.0123 

0.0171 

0.0297 

0.0736 

0.309 

2.55 

10.80 

41.9 

4.539 

0.0104 

0.0126 

0.0175 

0.0307 

0.0759 

0.319 

2.63 

11.2 

43.2 

4.307 

0.0114 

0.0137 

0.0193 

0.0339 

0.0840 

0.354 

2.93 

12.4 

47.9 

4.300 

0.0115 

0.0138 

0.0194 

0.0339 

0.0845 

0.355 

2.94 

12.4 

48.2 

4.085 

0.0124 

0.0151 

0.0213 

0.0375 

0.0933 

0.391 

3.26 

13.8 

53.5 

4.038 

0.0127 

0.0154 

0.0217 

0.0383 

0.0948 

0.402 

3.33 

14.1 

54.4 

3.800 

0.0140 

0.0172 

0.0244 

0.0430 

0.107 

0.451 

3.74 

16.0 

61.7 

3.714 

0.0146 

0.0179 

0.0255 

0.0450 

0.112 

0.470 

3.93 

16.8 

64.4 

3.691 

0.0148 

0.0182 

0.0257 

0.0455 

0.113 

0.479 

4.00 

17.0 

65.1 

3.589 

0.0156 

0.0191 

0.0272 

0.0480 

0.119 

0.502 

4.21 

18.0 

69.1 

3.404 

0.0171 

0.0211 

0.0301 

0.0531 

0.132 

0.560 

4.67 

20.0 

77.0 

3.400 

0.0171 

0 0211 

0.0302 

0.0533 

0.133 

0.560 

4.70 

20.0 

77.4 

3.230 

0.0187 

0.0232 

0.0333 

0.0589 

0.147 

0.617 

5.19 

22.2 

85.4 

3.142 

0.0196 

0.0243 

0.0350 

0.0622 

0.154 

0.653 

5.47 

23.3 

89.6 

3.076 

0.0203 

0.0253 

0.0364 

0.0645 

0.161 

0.678 

5.72 

24.4 

94.1 

2.936 

0.0221 

0.0276 

0.0397 

0.0706 

0.177 

0.746 

6.27 

26.9 

103. 

2.918 

0.0222 

0.0277 

0.0403 

0.0716 

0.178 

0.752 

6.35 

27.2 

104. 

2.871 

0.0229 

0.0287 

0.0414 

0.0736 

0.184 

0.776 

6.55 

28.0 

108. 

2.809 

0.0237 

0.0298 

0.0432 

0.0770 

0.192 

0.814 

6.84 

29.2 

113. 

2.723 

0.0251 

0.0316 

0.0461 

0.0818 

0.204 

0.863 

7.26 

31.3 

120. 

2.692 

0.0257 

0.0322 

0.0471 

0.0838 

0.207 

0.883 

7.46 

31.9 

122. 

2.584 

0.0277 

0.0348 

0.0508 

0.0910 

0.226 

0.964 

8.13 

34.8 

133. 

2.553 

0.0283 

0.0355 

0.0519 

0.0929 

0.231 

0.977 

8.30 

35.4 

136. 

2.403 

0.0313 

0.0398 

0.0583 

0.104 

0.260 

1.11 

9.34 

40.0 

154. 

2.349 

0.0328 

0.0413 

0.0607 

0.109 

0.272 

1.18 

9.75 

42.0 

161. 

2 269 

0.0350 

0.0442 

0.0652 

0.116 

0.291 

1.25 

10.5 

44.9 

173. 

2 153 

0.0384 

0.0488 

0.0718 

0.129 

0.321 

1.38 

11.7 

49.9 

192.0 

2.150 

0.0384 

0.0489 

0.0721 

0.130 

0.322 

1.39 

11.7 

50.0 

193.0 

2.043 

0.0423 

0.0538 

0.0794 

0.143 

0.355 

1.53 

12.9 

55.5 

213. 

1.988 

0.0444 

0.0566 

0.0838 

0.150 

0.376 

1.61 

13.6 

58.3 

225. 

1.857 

0.0502 

0.0644 

0.0954 

0.171 

0.430 

1.85 

15.6 

66.7 

258. 

1.846 

0.0506 

0.0653 

0.0964 

0.173 

0.432 

1.88 

15.8 

67.6 

261.0 

1.723 

0.0574 

0.0740 

0.110 

0.198 

0.496 

2.15 

18.1 

77.6 

299. 

1.702 

0.0588 

0.0757 

0.113 

0.202 

0.507 

2.20 

18.5 

79.4 

306. 

1.615 

0.0643 

0.0840 

0.125 

0.224 

0.562 

2.44 

20.6 

88.1 

340.0 

1.571 

0.0680 

0.0882 

0.132 

0.236 

0.594 

2.59 

21.8 

93.3 

361. 

1.520 

0.0720 

0.0944 

0.141 

0.251 

0.633 

2.77 

23.3 

100.0 

386. 

1 .459 

0.0776 

0.101 

0.152 

0.272 

0.684 

2.99 

25.2 

108. 

417. 

1.436 

0.0796 

0.105 

0.157 

0.281 

0.703 

3.09 

26.1 

112. 

432. 

1.362 

0.0877 

0.116 

0.174 

0.311 

0.7 S 3 

3.43 

28.9 

124. 

479. 


CONFIDENTIAI 





































254 


APPENDIX A 


Angular 

subtense 

I 

I M I N 

A L C 0 N T R 

AST (FOOT 

-LAMBERTS) 

of target 










(minutes) 

1,000 

100 

10 

1 

10" 1 

10~ 2 

10" 3 

10 -4 

10~* 

1.360 

0.0881 

0.116 

0.175 

0.312 

0.785 

3.45 

29.0 

125. 

480. 

1.292 

0.0966 

0.128 

0.193 

0.345 

0.868 

3.82 

32.2 

138. 

535. 

1.277 

0.0986 

0.131 

0.197 

0.352 

0.885 

3.90 

32.9 


544. 

1.201 

0.110 

0.148 

0.222 

0.395 

0.995 

4.45 

37.1 


617. 

1.175 

0.115 

0.154 

0.232 

0.413 

1.05 

4.58 

38.7 


643. 

1.167 

0.117 

0.155 

0.234 

0.419 

1.06 

4.63 

39.3 


652. 

1.135 

0.123 

0.164 

0.248 

0.442 

1.12 

4.91 

41.4 


687. 

1.077 

0.135 

0.182 

0.274 

0.491 

1.24 

5.48 

46.0 


766. 

1.075 

0.136 

0.182 

0.275 

0.492 

1.25 

5.50 

46.2 


770. 

1.021 

0.149 

0.200 

0.304 

0.542 

1.38 

6.09 

51.3 


851. 

0.9938 

0.157 

0.210 

0.319 

0.572 

1.45 

6.41 

53.6 


893. 

0.9726 

0.168 

0.219 

0.333 

0.596 

1.52 

6.67 

56.1 


941. 

0.9284 

0.177 

0.239 

0.364 

0.652 

1.66 

7.33 

61.6 


1030. 

0.9229 

0.180 

0.242 

0.368 

0.662 

1 .68 

7.41 

62.4 


1042. 

0.9078 

0.185 

0.250 

0.381 

0.682 

1.74 

7.66 

69.5 


1080. 

0.8880 

0.192 

0.260 

0.395 

0.714 

1.82 

7.98 

67.4 


1130. 

0.8613 

0.203 

0.277 

0.420 

0.758 

1.93 

8.49 

71.2 



0.8510 

0.209 

0.284 

0.432 

0.776 

1.98 

8.70 

73.3 



0.8170 

0.225 

0.306 

0.466 

0.841 

2.14 

9.44 

79.4 


1330. 

0.8075 

0.232 

0.313 

0.476 

0.859 

2.20 

9.66 

81.3 



0.7600 

0.258 

0.352 

0.538 

0.967 

2.48 

11.0 

92.0 



0.7428 

0.271 

0.367 

0.562 

1.01 

2.61 

11.5 

96.2 



0.7178 

0.290 

0.392 

0.598 

1.08 

2.79 

12.4 

104. 



0.6808 

0.320 

0.434 

0.664 

1.20 

3.10 

13.8 

116. 



0.6800 

0.322 

0.436 

0.667 

1.22 

3.12 


116. 



0.6460 

0.355 

0.480 

0.740 

1.34 

3.43 


129. 



0.6285 

0.374 

0.512 

0.783 

1.41 

3.64 


136. 



0.5873 

0.426 

0.582 

0.898 

1.61 

4.16 





0.5836 

0.432 

0.586 

0.912 

1.64 

4.21 





0.5447 

0.497 

0.676 

1.04 

1.88 

4.82 





0.5383 

0.507 

0.692 

1.07 

1.92 

4.96 





0.5106 

0.562 

0.766 

1.19 

2.14 

5.45 





0.4969 

0.596 

0.807 

1.26 

2.26 

5.77 





0.4806 

0.637 

0.871 

1.34 

2.42 

6.19 





0.4614 

0.687 

0.935 

1.46 

2.62 

6.68 





0.4539 

0.714 

0.975 

1.50 

2.71 

6.92 





0.4307 

0.787 

1.08 

1.67 

3.01 

7.67 





0.4300 

0.793 

1.08 

1.68 

3.01 

7.74 





0.4085 

0.881 

1.20 

1.85 

3.34 

8.52 





0.4038 

0.902 

1.23 

1.90 

3.42 

8.70 





0.3800 

1.02 

1.38 

2.14 

3.85 

9.86 





0.3714 

1.06 

1.44 

2.24 

4.04 

10.4 





0.3691 

1.08 

1.46 

2.27 

4.09 

10.5 





0.3589 

1.14 

1.55 

2.40 

4.32 

11.1 





0.3404 

1.28 

1.73 

2.68 

4.82 

12.4 





0.3400 

1.28 

1.73 

2.68 

4.83 

12.4 





0.3230 

1.40 

1.91 

2.96 

5.31 

13.7 





0.3142 

1.49 

2.02 

3.14 

5.62 






0.3076 

1.55 

2.11 

3.26 

5.85 






0.2936 

1.70 

2.32 

3.58 

6.43 






0.2918 

1.73 

2.33 

3.63 

6.53 






0.2871 

1.77 

2.42 

3.76 

6.74 






0.2809 

1.86 

2.54 

3.91 

7.02 






0.2723 

1.99 

2.69 

4.17 

7.50 

19.3 





0.2692 

2.03 

2.75 

4.27 

7.67 






0.2584 

2.19 

2.98 

4.63 

8.32 






0.2553 

2.25 

3.07 

4.74 

8.55 






0.2403 

2.52 

3.43 

5.36 

9.55 






0.2349 

2.66 

3.62 

5.60 

10.0 







CONFIDENTIAL 





















APPENDIX A 


255 


Angular 
subtense 
of target 
(minutes) 

L 

I M I N A L CO 

N T R A 

1,000 

100 

10 

1 

0.2269 

2.86 

3.88 

6.01 

10.8 

0.2153 

3.16 

4.28 

6.68 

12.0 

0.2150 

3.19 

4.32 

6.68 

12.0 

0.2043 

3.53 

4.78 

7.40 

13.3 

0.1988 

3.72 

5.04 

7.81 

14.1 

0.1857 

4.26 

5.76 

8.97 


0.1846 

4.32 

5.82 

9.06 


0.1723 

4.96 

6.67 

10.3 


0.1702 

5.08 

6.86 

10.6 


0.1615 

5.62 

7.62 

11.9 


0.1571 

5.96 

8.04 

12.5 


0.1520 

6.38 

8.61 

13.4 


0.1459 

6.91 

9.31 

14.5 


0.1436 

7.14 

9.66 



0.1362 

7.74 

10.7 



0.1360 

7.95 

10.7 



0.1292 

8.83 

11.9 




( F O O T - L 


IQ " 2 


A M B E R T S ) 


10~ 3 


1(T 4 


10“ 5 


CONFIDENTIAL 


























GLOSSARY 


A AFT AC. Army Air Forces Tactical Air Center. 

Achromatic Color. White, gray, or black. 

Ad Hoc Committee. A fact-finding committee whose exist¬ 
ence terminated automatically after its report had been 
made. 

Adaptation. Process by which the eye achieves optimum 
perceptual capacity for a given set of lighting conditions. 

Adaptation Level. The brightness of a uniform field of 
view to which the eye attains a given state of adapta¬ 
tion. 

Apparent Contrast. The contrast of an object as it appears 
to a distant observer. 

ASDevLant. Anti-Submarine Development Detachment, 
Air Force, U. S. Atlantic Fleet. 

Average Reflectance. The reflectance of a uniform surface 
perpendicular to the line of sight at the target having a 
contrast against the sky equal to the integrated contrast 
of the target. 

|3. Atmospheric attenuation coefficient. (Typical unit: re¬ 
ciprocal meters.) 

Black Widow Finish. Antisearchlight camouflage for air¬ 
craft described in Chapter 7. (Origin of code name: first 
used on P61, Black Widow nightfighters.) 

Bowditch’s Rule. “The distance from an observer to the 
horizon, expressed in miles, equals the square root of 3/2 
the height of the observer’s eye above the sea measured in 
feet.” 

[»- /!"»]■ 

(This rule is based upon trigonometric approximations 
which render its predictions inaccurate except for small 
values of H .) 

BuAer. Bureau of Aeronautics, U. S. Navy. 

BuOrd. Bureau of Ordnance, U. S. Navy. 

BuShips. Bureau of Ships, U. S. Navy. 

Chromatic Color. A color other than white, gray, or black. 

Chromatic Contrast. Color contrast. 

Chromaticity. Those properties of a color described by its 
dominant wavelength and purity. 

Coffin Paint. Matte black paint of exceptionally law dif¬ 
fuse reflectance, developed under Contract OEMsr-697. 
(Origin of name: A substitute for “coffin paper” produced 
originally for coffin manufacturers but used as an anti¬ 
reflection lining for high-grade optical instruments.) 


Color Contrast. Departure in chromaticity of a target from 
its background. 

Contained Shadow. A black area within the outline of a 
ship or plane produced by the shadow of some overhang¬ 
ing structure. 

Contrast. The fractional difference in brightness between 
an object and its background. 

Counter Shading. A method of camouflage painting 
whereby the reflectance is graded in a manner inversely 
related to the illumination of the surface. In general, dark 
paint is used on upper surfaces and light paint is used on 
under surfaces. 

Critical Point. The point of closest approach along the 
path of flight of a bomber to its target at which the 
bombardier must be able to see the target in order to drop 
the bomb on it. 

Daylight Visual Range. That distance at which a large 
dark object on the horizon is just recognizable against 
the sky background. 

Demagnification. A reduction in the apparent size of ob¬ 
jects. (Example: The effect of a telescope looked through 
in reverse direction.) 

Densitometer. Apparatus for measuring the transmittance 
or “density” of a photographic film. 

Density (Photographic). A measure of the blackness of a 
photographic film. (Quantitative definition: Density = 
log 10 1/transmittance.) 

Density (Physical). Mass per unit volume. (Typical unit: 
kilograms per cubic meter.) 

Desaturation. Reduction in purity. 

Diffuse Reflection. Light reflected in all directions (as by 
a sheet of blotting paper). 

Dominant Wavelength (of a Color). The wavelength of 
that monochromatic light which, when mixed in proper 
proportion with white light, will match the chromaticity 
of the sample. 

EA.C. Equivalent achromatic contrast. 

Effective Inherent Contrast. The inherent contrast of a 
uniform target of the same size and shape as a given non- 
uniform target having the same liminal target distance. 

Effective Projected Target Area. (See Section 5.3.1.) 

Equivalent Achromatic Contrast. That brightness con¬ 
trast which produces the same acuity as a color contrast 
(see Section 3.3). 

Flatting Agent. A material added to paint in order to 
produce a matte surface. (Example: asbestene.) 


CONFIDENTIAL 


257 


258 


GLOSSARY 


Foot-lambert. A unit of brightness. One foot-lambert is the 
brightness of a perfectly diffusing surface emitting or re¬ 
flecting one lumen per square foot. 

Form Factor. The ratio of the liminal contrast of a non¬ 
circular target to the liminal contrast of a circular target 
of equal area. 

Glomb. A bomb-carrying, remote-control glider. 

Goniophotometer. A laboratory instrument for measuring 
the reflectance of materials for any angle of incidence and 
observation. 

Goniophotometric Curve. A plot of the readings of a 
goniophotometer over a range of angles of incidence or 
observation. 

Gonioreflectance. The reflecting properties of a surface as 
defined by a goniophotometric curve. 

Gray Scale. A graded series of gray panels of known re¬ 
flectance. 

Haze Box. A viewing device capable of simulating the 
effect of atmospheric haze. 

High Level. Photopic brightness level. 

I. C.I. International Commission on Illumination. 

Illumination Ratio. Term sometimes used as a synonym 
for sun-ratio. 

Infrared. In this volume the term “infrared” has been used 
to denote that portion of the electromagnetic spectrum 
having wavelengths longer than visible light, but short 
enough to be detected by infrared Aero film (700 to 950 
millimicrons). 

Inherent Contrast. The contrast of an object as seen 
nearby. 

Inherent Integrated Contrast. Integrated contrast of a 
target seen nearby. 

Inherent Internal Contrast. Internal contrast of a target 
as seen nearby. 

Integrated Contrast. An average of the internal contrasts 
of a patterned target weighted in accordance with the 
area of the pattern elements. 

Internal Contrast. Contrast between parts of a patterned 
target. 

Ishihara Test. A common test for color blindness. 

J. O.S.A. Journal of the Optical Society of America. 

Landolt Ring. A broken ring pattern. 

Lapse Rate. The variation of temperature with altitude. 

LBE. Glide bomb Pratt-Read (“Glomb”). 

LBT. Glide bomb Taylorcraft (“Glomb”). 

Liberator. Four-motored U. S. bomber (Army: B-24. Navy: 
PB4Y). 


Liminal Contrast. Value of contrast for which the prob¬ 
ability of an observer making a correct response is 50 per 
cent greater than chance. 

Liminal Target Distance. That distance at which a target 
is visually detectable with a probability 50 per cent 
greater than chance. 

Low Level. Scotopic brightness level. 

Meteorological Range. That horizontal distance for w r hich 
the transmittance of the atmosphere is 2 per cent (see 
Section 2.2.5). 

Microdensitometer. Apparatus for measuring the density 
of very small areas of a photographic film. 

Monochromatic Light. A narrow band from the visible 
spectrum within which the range of wavelength is so 
small that the physical phenomena under consideration 
show no significant wavelength dependence. 

Munsell Paper. Special colored papers sold by the Munsell 
Color Company, Baltimore, Maryland, for use as color 
standards. 

Nomographic Charts. (Synonym: alignment charts). Charts 
in which the relation between three or more variables are 
expressed by a series of scales and lines so arranged that 
an unknown value of one of the variables can be deter¬ 
mined from known values of the others by establishing 
one or more straight lines across the chart. 

Optical Equilibrium. (See Section 2.2.1.) 

Optical Slant Range. (See Section 2.2.3.) 

Optically Homogeneous Atmosphere. An atmosphere 
wherein 3, a, and q have the same values at all points 
along the line of sight. 

Ostwald Paper. Papers the colors of which were specified 
in the Ostwald color notation. 

PBM. Patrol bomber Martin. 

P.D.P. Passive Defense Project, Work Projects Administra¬ 
tion, Project No. 22242. 

Photopic. Pertaining to the properties of the human eye 
when adapted to full daytime levels of brightness. 

Purity (of a Color). A measure of the proportion in which 
w T hite light and monochromatic light of the dominant 
wave length must be mixed in order to match the chro- 
maticity of the color. Achromatic colors have zero purity. 
Monochromatic light is considered 100 per cent pure. 

Purkinje Effect. Shift in the spectral sensitivity of the 
human eye at reduced levels of brightness. (See “Prin¬ 
ciples of Optics,” Hardy and Perrin, p. 191.) 

q. Luminous density (see Section 2.3.2). (Typical unit: 
lumjoules per cubic meter.) 

Reflectance. Ratio of the light reflected by an object to 
the light incident upon it. 


CONFIDENTIAL 



GLOSSARY 


259 


Sky-Ground Ratio. The ratio of the brightness of the sky 
in particular directions (see Figure 17, Chapter 2) to the 
brightness of the ground (see Section 2.3.6). 

Slant Range. The distance from an aircraft to its target 
along a slanting path of sight. 

Snellen-type. Typical test chart used by oculists. 

Solar Altitude. The angular elevation of the sun above the 
horizon. 

Solar Depression. The negative of solar altitude. After 
sunset, values of solar depression are positive, since solar 
altitude assumes negative values. 

Spectral Reflectance. Reflectance in terms of monochro¬ 
matic light. 

Specular Reflection. The mirror-like reflection from a 
smooth surface. 

Spectrogeograph. A spectrograph for aerial use in deter¬ 
mining the reflectance of natural terrains and the optical 
properties of the atmosphere (see Chapter 6). 

Spectrophotometer. Laboratory instrument for measuring 
the reflectance of materials, wave length by w r ave length. 

Spot Track. Imaginary circle of fixed diameter around 
which targets were presented by projection during the 8- 
position experiments at the Tiffany Foundation. 

Standard Atmosphere. (See Section 2.3.2.) 


Sun-Ratio. The ratio of the illumination on a vertical sur¬ 
face facing the sun to the illumination on a vertical sur¬ 
face facing away from the sun. 

Target Point. The point on a nomographic visibility chart 
which is determined by the effective projected target area 
and the optical slant range from the target to the critical 
point. 

Telephotometer. A photometer for measuring the apparent 
brightness of distant objects. 

Tone Down. Camouflage accomplished by giving the tar¬ 
get a dark color. 

Tone Down Limit. The greatest value of meteorological 
range for which a target can fee concealed from an obser¬ 
ver at the critical point by tone down measures. 

Transmissometer. An apparatus for measuring the trans¬ 
mittance of the atmosphere. 

Transmittance. Ratio of the light transmitted by an ob¬ 
ject to the light incident upon it. 

Troffer. A trough containing lamps. 

Variac. Trade name for an auto-transformer having an ad¬ 
justable voltage output. 

“Visibility.” See Section 2.2.5. 

Yehudi. Code name for use in unclassified correspondence 
concerning the camouflage of aircraft by means of beams 
of light projected into the eyes of the enemy. 


CONFIDENTIAL 











BIBLIOGRAPHY 


The numerical symbol which appears after many of the bibliographical entries indicates that the document 
listed has been microfilmed. The classification system for the microfilm record is explained in the microfilm index 
volume of the Summary Technical Report. Inquiries regarding the availability of the microfilm index volume and 
of the microfilm should be addressed to the Army or Navy agency listed on the reverse of the half-title page. 


1. Report of NDRC ad hoc Committee on Camouflage, 

Feb. 25, 1942, p. 3. Div. 16-200-MI 

la. Ibid., p.6. 

2. The Preparation and Properties of Chlorophyll Paints, 
Kenneth V. Thimann and David Kaufman, OEMsr-551, 
OSRD 1026, Harvard University, Oct. 31, 1942. 

Div. 16-220-MI 

3. “Construction of the General Electric Recording Spec¬ 
trophotometer,” J. L. Michaelson, Journal of the Optical 
Society of America, Vol. 28, No. 10, October 1938, pp. 
365-371. 

4. “History of the Design of the Recording Spectropho¬ 
tometer,” Arthur C. Hardy, Journal of the Optical So¬ 
ciety of America, Vol. 28, No. 10, October 1938, pp. 
360-364. 

5. “A New Recording Spectrophotometer,” Arthur C. 
Hardy, Journal of the Optical Society of America, Vol. 
25, No. 9, September 1935, pp. 305-311. 

6. Photography by Infrared, Walter Clark, John Wiley & 
Sons, Inc., New York, N. Y., 1939, p. 168. 

7. The Photographic Process, Julian Ellis Mack and Miles 
J. Martin, McGraw-Hill Book Company, Inc., New 
York, N. Y., 1939, p. 436. 

8. Optionic Instruments for the Study of Camouflage, Re¬ 
port 12, OEMsr-697, OSRD 6556, Section 16.3-12, Inter¬ 
chemical Corp., Aug. 25, 1945, p. 6. Div. 16-272-MI 
8a. Ibid., p. 17. 

8b. Ibid., p. 66. 

8c. Ibid., p. 51. 

8d. Ibid., p. 63. 

8e. Ibid., p. 44. 

9. The Principles of Optics, Arthur C. Hardy and Fred H. 
Perrin, McGraw-Hill Book Company, Inc., New York, 
N. Y., 1932, p. 282. 

9a. Ibid., p. 270. 

10. Gloss, Seibert Q. Duntley, Research Paper 4, PDP 
Physical Measurement Laboratory, May 26, 1941. 

Div. 16-210-MI 

10a. Ibid., p. 4. 

11. A Spectrograph for Aerial TJse, The Spectrogeograph, 
Arthur C. Hardy, Report 5, OEMsr-717, OSRD 5444, 
Section 16.3-5, Eastman Kodak Co., Dec. 9, 1944. 

Div. 16-271-MI 


12. Visibility in Meteorology, W. E. K. Middleton, The 
University of Toronto Press, Toronto, Ont., 1941. 

12a. Ibid., p. 56. 

12b. 7hfd.,p.39. 

13. “The Mathematics of Turbid Media,” Seibert Q. Dunt¬ 
ley, Journal of the Optical Society of America, Vol. 33, 
No. 5, May 1943, pp. 252-257. 

14. Reflectance of Natural Terrains, Report 10, OEMsr- 
597, OSRD 6554, Section 16.3-10, The Louis Comfort 
Tiffany Foundation, Sept. 14, 1945. Div. 16-240-M2 

15. Calibration and Use of the Spectrogeograph, Seibert Q. 
Duntley, Report 11, OEMsr-597, Section 16.3-11, The 
Louis Comfort Tiffany Foundation, Sept. 24, 1945. 

Div. 16-271-M2 

16. Transient Color Phenomena in a Desert, Report 8, 
OEMsr-597, OSRD 6552, Section 16.3-8, The Louis 
Comfort Tiffany Foundation, Apr. 10, 1945. 

Div. 16-230-M2 

17. Water Camouflage, B. T. Mesier, Report 1, OEMsr- 
726, Research Project PDRC-728, Section 16.3-1, Amer¬ 
ican Cyanamid Co., June 30, 1943. Div. 16-261-MI 

18. Camouflage Finishes and Related Problems, Report 13, 

OEMsr-697, OSRD 6557, Section 16.3-13, Interchemical 
Corp., Sept. 24, 1945. Div. 16-220-M2 

18a. Ibid., p. 161. 

18b. Ibid., p. 116. 

18c. Ibid., p. 7. 

18d. Ibid., p.133. 

18e. Ibid., p. 153. 

18f. Ibid., p. 129. 

18g. Ibid., p. 80. 

18h. Ibid., p. 176. 

19. Camouflage of Sea-Search Aircraft, Yehudi Project, 
Report 2, OEMsr-597, OSRD 3816, Section 16.3-2, The 
Louis Comfort Tiffany Foundation, June 1, 1944. 

Div. 16-262-MI 

20. Camouflage of a Glomb, Seibert Q. Duntley, Report 6, 

OEMsr-1459, OSRD 5371, Section 16.3-6, Pratt, Read 
and Co., Inc., Aug. 31, 1945. Div. 16-263-MI 

21. Aircraft Camouflage Illumination, Serial 0109, Navy 
Department, Antisubmarine Development Detachment, 
Air Force, U. S. Atlantic Fleet, Mar. 14, 1944. 


CONFIDENTIAL 


261 


262 


BIBLIOGRAPHY 


22. Influence of Color Contrast on Visual Acuity, Report 3, 37. Visibility of Targets in Relation to Night Screening, 


OEMsr-1070, OSRD 4541. Section 16.3-3, Eastman 
Kodak Co., Nov. 1, 1944. Div. 16-230-MI 

23. Visibility of Targets (Record of Research) , Vols. I-V, 

The Louis Comfort Tiffany Foundation, March 1943- 
September 1945. Div. 16-250-M2 

23a. Ibid., p. 149. 

23b. Ibid., p. 2589. 

24. Visibility of Targets, Report 7, OEMsr-597, OSRD 

6401, Section 16.3-7, The Louis Comfort Tiffany Founda¬ 
tion, Oct. 1, 1945. Div. 16-250-MI 

25. Development of a Transmissometer for Determining 
Visual Range, C. A. Douglas and L. L. Young, National 
Bureau of Standards, Technical Report 47, U. S. De¬ 
partment of Commerce, CAA, February 1945. 

25a. Ibid., p. 14. 

26. “A New Method for Photographic Spectrophotometry,” 
Lloyd A. Jones, Journal of the Optical Society of Amer¬ 
ica, Vol. 10, No. 5, May 1925, pp. 561-572. 

27. Encyclopaedia Britannica, Vol. 3, 1945, p. 129. 

28. Psychometric Methods, J. P. Guilford, McGraw-Hill 
Book Company, Inc., New York, N. Y., 1936, p. 170. 

29. “Energy and Vision,” S. P. Langley, The London, Edin¬ 
burgh and Dublin Philosophical Magazine and Journal 
of Science, Ser. 5, Vol. 27, January 1889, pp. 1-23. 

30. “Untersuchungen am Lummer-Pringsheimschen Spek- 
tralflickerphotometer,” Hedwig Bender, Zeitschrift fur 
Sinnesphysiologie, Vol. 50, 1919, pp. 1-41. 

31. “Die Irradiation als Ursache Geometrische Optischer 
Tauschungen.” Alfred Lehman, Pfluger’s Archiv fur 
die Gesamte Pliysiologie des Menschen und der Tiere, 

Vol. 103, 1904. p. 90. 

32. Untersuchungen zur Gegenstandstheorie und Psychol¬ 
ogic, compiled by R. von H. Meinong, 1904, pp. 449- 
472, “Die Verschobene Schachbreppfigur,” Vittorio Be- 
nussi and Wilhelmine Liel. 

r 

33. “Uber das Verhalten Farbiger Formen bei Helligkeits- 
gleichheit von Figur and Grund,” Susanne Liebmann, 
Psychologische Forschungen, Vol. 9, 1927, pp. 300-353. 

34. “Colour and Organization,” Kurt Koffka and M. R. 
Harrower. Psychologische Forschungen, Vol. 15, 1931, 
pp. 145-192, pp. 193-275. 

35. “Uber die Anderung des Sehvermogens durch Farbige 
Schutzglaser,” H. Hartinger and F. Schubert, Klinische 
Monatsblatter fur Augenheilkunde, Vol. 105, 1940, p. 
337. 

36. “Einfluss Gebrauchlicher Farbiger Schutzglaser auf das 
Sehvormogen durch Anderung des Leuchtdichtekon- 
trasten, des Farbtonkontrastes und der Sattigung,” Cl. 
Schaefer, W. Kliefoth, and Th. von Wolff, Zeitschrift 
fur Technische Physik, Vol. 24, No. 6, 1943, pp. 125-140. 


G. O. Langstroth, H. F. Batho, M. W. Johns, J. L. 
Wolfson, E. H. McLaren, and D. D. Levi, Canadian 
Report III-1-1230, Project CE 128, Department of 
Physics, University of Manitoba, Winnipeg, Man., Oct. 
15. 1943. 

38. The Measurement of Visual Acuity, R. J. Lythgoe, 
Medical Research Council Special Report Series, No. 
173. London, H. M. Stationery Office, 1932. 

39. Report on Nomographic Chart, R. D. Douglass, The 
Louis Comfort Tiffany Foundation, MIT. 

Div. 16-281-MI 

40. “Atmospheric Limitations on the Performance of Tele¬ 
scopes,” Arthur C. Hardy, Journal of the Optical So¬ 
ciety of America, Vol. 36, No. 5. May 1946. pp. 283- 
287. 

41. “The Stiles-Crawford Effect and the Design of Tele¬ 
scopes,” Donald H. Jacobs, Journal of the Optical So¬ 
ciety of America, Vol. 34, No. 11, November 1944, p. 
694. 

42. “A Brightness Meter,” Matthew Luckiesh and A. H. 
Taylor. Journal of the Optical Society of America, Vol. 
27, No. 3, March 1937, p. 132. 

43. An Integrating Contrast Photometer, The High Hill 

Project, Seibert Q. Duntley, Report 9. OEMsr-597, 
OSRD 6553. Section 16.3-9, The Louis Comfort Tiffany 
Foundation, July 24, 1945 Div. 16-273-MI 

44. American Practical Navigator, Nathaniel Bowditch, 
U. S. Hydrographic Office, No. 9, 1917, p. 508. 

45. “The Use of Modulated Lamps in Photometry” (Ab¬ 
stract), Seibert Q. Duntley, Journal of the Optical So¬ 
ciety of America, Vol. 31, No. 6, June 1941, p. 460. 

46. “A Simple Method for Investigating the Optical Modu¬ 
lation of a Gaseous Conduction Lamp” (Abstract), 
D. B. Cameron and Seibert Q. Duntley. Journal of the 
Optical Society of America, Vol. 31, No. 6, June 1941, 
p. 463. 

47. “A Modulated Lamp Densitometer” (Abstract), Seibert 
Q. Duntley and John E. Tyler, Journal of the Optical 
Society of America, Vol. 31, No. 6, June 1941, p. 
461. 

48. Technical Memoranda, Seibert Q. Duntley to W. P. 

Greenwood, Mar. 13, 1944. Div. 16-240-MI 

49. “Construction and Test of a Goniophotometer,” Parry 
Moon and Jacques Laurence, Journal of the Optical 
Society of America, Vol. 31. No. 2. February 1941, pp. 
130-139. 

50. Handbook of Colorimetry, Arthur C. Hardy, The Tech¬ 
nology Press, MIT. 1936. 

51. “Estimation of Chromaticity Differences and Nearest 
Color Temperature on the Standard 1931 ICI Colori- 


CONFIDENTIAL 




BIBLIOGRAPHY 


263 


metric Coordinate System,” Deane B. Judd, Journal of 54. “On the Geometry of Color Space,” David L Mac- 
the 0ptical Societ y °f America, Yol. 26, No. 11, Novem- Adam, Journal of The Franklin Institute, Yol. 238 Sep- 
ber 1936, pp. 421-426. tember 1944, pp. 195-210. 


52. “Daylight Illumination on Horizontal, Vertical and 
Sloping Serfaces,” Herbert K. Kimball and Irving F. 
Hand, Monthly Weather Review, Vol. 50, 1922, pp. 65- 
625. 

53. “The Duration and Intensity of Twilight,” Herbert H. 
Kimball, Monthly Weather Review, Vol. 66, 1938, pp. 
279-286. 


55. Aerial Haze and Its Effect on Photography from the 
Air, Monograph Number 4, Research Laboratories of 
the Eastman Kodak Co., August 1923. 

56. Untersuchungen fiber die Spektrale Zusammensetzung 
der bei Luftaufnahmen Wirksamen Strahlung,” R. 
Schimpf and C. Aschenbrenner. Zeitschrift fur Ange- 
wandte Photographie, Vol. II, No. 3, June 1940, pp. 41- 
45. 


CONFIDENTIAL 






OSRD APPOINTEES 


Division 16 

Chief 

George R. Harrison 


Deputy Chiefs 

Paul E. Klopsteg 


Herbert E. Ives 


H. R. Clark 
J. S. Coleman 


Consultants 


Technical Aides 


H. K. Stephenson 


Richard C. Lord 


F. E. Tuttle 


C. A. Federer, Jr. 
Richard C. Lord 


Arthur C. Hardy 
Herbert E. Ives 
Paul E. Klopsteg 
Brian O’Brien 
F. E. Tuttle 


Members 

0. S. Duffendack 
Theodore Dunham, Jr. 

E. A. Eckhardt 
Harvery Fletcher 
W. E. Forsythe 


Section 16.1 
Chief 

Theodore Dunham, Jr. 


G. W. Morey 
F. L. Jones 


Consultants 


H. F. Mark 
H. F. Weaver 


Technical Aides 

Lillian Elveback S. W. McCuskey 

H. F. Weaver 


Ira S. Bowen 
W. V. Houston 


Members 


F. E. Wright 


R. R. McMath 
G. W. Morey 


264 


CONFIDENTIAL 


Section 16.2 


Chief 

Brian O’Brien 

Consultants 

W. R. Brode V. K. Zworykin 


Technical Aide 

Chas. E. Waring 


Members 

W. E. Forsythe Julian H. Webb 

Harvey E. White 


Section 16.3 

Chief 

Arthur C. Hardy 


Consultants 

Lewis Knudson Edward R. Schwarz 

Parry H. Moon 

Technical Aides 

S. Q. Duntley Arthur W. Kenney 

Ernest T. Larson 


Members 

Edwin G. Boring L. A. Jones 

F. C. Whitmore 


Section 16.4 

Chief 

0. S. Duffendack 

Consultants 

W. L. Enfield H. G. Houghton, Jr. 

W. H. Radford 

CONFIDENTIAL 


265 


H. S. Bull 

Technical Aides 

James S. Owens 

Winston L. Hole 

Alan C. Bemis 
Saul Dushman 

Members 

H. G. Houghton, Jr. 
George A. Morton 


Section 16.5 


W. E. Forsythe 

Chiefs 

Herbert E. Ives 

W. E. Forsythe 

Deputy Chiefs 

Brian O’Brien 

E. Q. Adams 

Consultants 

A. C. Downes 

Technical Aides 

William Herriott 

Val E. Sauerwein 

John T. Remey 

D. W. Bronk 

A. C. Hardy 
Theodore Matson 
A. H. Pfund 

Members 

W. B. Rayton 

A. B. Simmons 

G. F. A. Stutz 
Harvey E. White 


V. K. Zworykin 


266 


CONFIDENTIAL 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECTS OF 

CONTRACTS FOR SECTION 16.3 

Contract No. 

Contractor 

Subject 

OEMsr-551 

Harvard University, 

Cambridge, Massachusetts 

. . studies and investigations in connection with the extraction of 
chlorophyll from plant sources and its use as a pigment. . . 

OEMsr-597 

Trustees of the Louis Comfort 
Tiffany Foundation, Oyster 
Bay, Long Island, New York 

“. . . perform certain camouflage field studies. . . 

OEMsr-697 

Interchemical Corporation, 

New York, New York 

. . studies and investigations of characteristics of camouflage paints, 
develop noncritical substitutes, improve and simplify procedure 
in field practice, and develop and construct such special appara¬ 
tus as may be requested by the Contracting Officer or an author¬ 
ized representative, for use in camouflage field studies. . . .” 

OEMsr-717 

Eastman Kodak Company, 
Rochester, New York 

. . studies and experimental investigations in connection with the 
design and construction of an instrument and the development 
of techniques for its use in measuring the quantity and spectral 
quality of radiant energy from natural daytime sources reaching 
an aeroplane during flight. . . .” 

OEMsr-726 

American Cyanamid Company, 

30 Rockefeller Plaza, 

New York, New York 

“. . . studies and experimental investigations in connection with the 
camouflaging of stationary or slowly moving bodies of water by 
thin surface films. . . .” 

OEMsr-1020 

Cornell University, 

Ithaca, New York 

“. . . studies, experimental investigations, and field tests in connection 
with the uses of plants and plant materials in camouflage. . . .” 

OEMsr-1070 

Eastman Kodak Company, 
Rochester, New York 

“. . . conduct a quantitative study of the effect on visibility of differ¬ 
ences in chromaticity. . . .” 

OEMsr-1459 

Pratt, Read & Company, Inc., 
Deep River, Connecticut 

“. . . studies and experimental investigations in connection with the 
design and installation on a Navy LBE aircraft of special camou¬ 
flage equipment. . . 


CONFIDENTIAL 


267 







SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Office of the Executive 
Secretary, OSRD, from the War or Navy Department through either 
the War Department Liaison Officer for NDRC or the Office of Research 
and Inventions (formerly the Coordinator of Research and Develop¬ 
ment), Navy Department. 


Service 

Project 

Number 


Subject 


Army Projects 


AC-45 


Development of Equipment for Rendering an Aircraft Less Visible to an Observer on the 


CE-24 


CE-26 


CE-25 


Surface of the Earth 
Fundamental Optics 
Paints and Pigments 
Color Transients 


Navy Projects 


NA-188 


Yehudi 


NS-147 


Ship Camouflage 


268 


CONFIDENTIAL 







INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. For access 
to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


Absorption of light by atmosphere, 20 
Ad Hoc Committee on Camouflage, 
NDRC, 3-4 

Adhesive, camouflage, 10-11 
American Cyanamid Company, 9 
Anti-searchlight camouflage, 11-12, 
242-248 

Apparent brightness 
see Brightness, apparent 
Atmosphere, standard 
defined, 29 

density in molecules per unit vol¬ 
ume, 30-31 

Atmospheric attenuation coefficient, 
20 

Atmospheric haze 

effect on meteorological range, 138- 
194 

effect on visibility, 19-32 
value in camouflage engineering, 221 
Atmospheric scattering, 6-8 
Atmospheric stratification, 138-194 
Attenuation coefficient of light, at¬ 
mospheric, 20 

B-17 Flying Fortress, spectrograph in¬ 
stallation, 202 

B-24 Liberator bomber, camouflage 
studies 

Black Widow finish, 242 
coffin paint, 242 
Yehudi camouflage, 14, 227-230 
Backgrounds other than horizon sky, 
32 

contrast formula, 126 
nomographic visibility charts, 126 
Black targets, visibility studies, 22-26 
Black Widow finish, 11-12, 242-250 
application, 248-249 
flight tests, 243, 247 
formulation, 247-248 
maintenance, 249-250 
Brightness, apparent 
attenuation of brightness differ¬ 
ences, formula, 31 
experimental studies, 22-26 
of an object at range x, formulas, 
20-21 

of the earth, 31 
of the horizon, formula, 20 
optical equilibrium condition for¬ 
mula, 20 

variation along slant paths, 31-32 
Brightness contrast 


see also Liminal brightness contrast 
against backgrounds other than 
horizon sky, 31-32 
apparent brightness contrast for¬ 
mulas, 21-22, 74 
attenuation with distance, 22 
average reflectance, 134 
black and white targets, 22-26 
definition, 134 
effect of subtense angle, 57 
effect of target shape, 33 
effect on liminal target distance, 74 
effect on visual acuity, 71-72 
effective inherent contrast. 130, 131 
inherent contrast formulas, 21-22, 74 
inherent integrated contrast, 130, 
131-135 

liminal contrast of circular targets, 
33-59, 251-255 

measuring instruments, 131-135 
nomographic charts, 78 
value equivalent to color contrast, 
61-64 

variation along a slant path, for¬ 
mula, 31-32 

Camouflage 
definition, 3 

of Navy guided missiles, 241 
of sea search aircraft, 225-241 
of ships, 13-14 
of water surfaces, 9 
Camouflage, Ad Hoc Committee on, 
3-4 

Camouflage adhesive, 10-11 
Camouflage countermeasures 
bifocal goggles, 12 
camouflage detection, 12 
to Yehudi camouflage, 234, 239 
Camouflage engineering 
basic requirements, 216-221 
choice of materials, 221 
peacetime applications, 221 
typical problem, 217-221 
Camouflage measures 
brightness contrast, 221 
color contrast, 221 
paints, 4, 9-12, 213, 242-250 
self-healing, self-spreading films, 9 
tone-down, 220 
Yehudi principle, 225-241 
Camouflage paints, 9-12 
Black Widow finish, 11-12, 242-250 
chlorophyll paint, 4, 10 

CONFIDENTIAL 


coffin paint, 11, 242 
emulsifiable paints, 10 
foliage-simulating pigments, 10 
glossy black, 11-12, 242-250 
high-reflectance white paint, 11 
infrared-bright green paints, 4 
matte surface paints, 10. 11, 242 
paint concentrates, 10 
simplification of palette, 9 
temperature-sensitive pigments, 10 
texture-simulating, 213 
Camouflage section, NDRC, 4 
“Ceilings” represented on optical slant 
range diagrams, 138 
Chlorophyll paint, 4, 10 
Chromatic aberration of the eye, ef¬ 
fect on visual acuity, 62-64 
Claray high reflectance white paint, 11 
“Coffin paint”, 11, 242 
Color contrast 

as camouflage measure, 221 
early investigation, 59-61 
effect on liminal target distance, 131 
effect on visual acuity, 71-72 
equivalent achromatic contrast, 61- 
64, 65-66 

measuring apparatus, 66-70 
observers, 71 

Color of underwater terrain, 8, 210 
Color transients in desert terrain, 8-9 
Colored lights, recognition threshold, 
13 

Contrast, brightness 
see Brightness contrast 
Contrast, color 
see Color contrast 

Contrast, combined color and bright¬ 
ness, 64-66 

effect on acuity, formula, 71-72 
formula, 65 

nomographic chart, 221 
Contrast photometer for field use, 134 
Countermeasures, camouflage 
bi-focal goggles, 12 
camouflage detection, 12 
to Yehudi camouflage, 234, 239 

Daylight visual range 
defined, 26-27 

directional variations, formulas, 27- 
28 

in terms of meteorological range, 
218 

Densitometer, photoelectric, 205-207 

269 


270 


INDEX 


Density of standard atmosphere, 30-31 
Desert terrain, color transients, 8-9 
Dichroic filters for camouflage de¬ 
tection, 12 

Diffraction of light around target, no¬ 
diffusion effect, 26 

Diffuse reflectance, camouflage paints 
Black Widow finish, 243, 247 
coffin paint, 242 

Eastman Kodak Company, 59-72 
Edge effect, nonexistent, 26 
Electrical Testing Laboratories, 50 
Emulsifiable paints, 10 
Equilibrium, optical 
along a slant path, 31 
along horizontal paths, 20 
extent, 20 
formula, 20 

Equivalent achromatic contrast, 61-64 
constancy with adaption level, 65-66 

Films, self-spreading, for water cam¬ 
ouflage, 9 

Filters, dichroic, for camouflage de¬ 
tection, 12 

Filters for color temperature correc¬ 
tion, 234 

Foliage-simulating pigments, 10 
Form factor of targets, 59 
Formulas 

apparent and inherent contrast, 21- 
22, 31-32 

apparent brightness, 20-21, 31 
atmospheric attenuation coefficient, 

20 

attenuation of brightness differ¬ 
ences, 31 

contrast formula, backgrounds other 
than horizon sky, 126 
daylight visual range, directional 
variations, 27-28 

effective projected target area, 194 
equivalent achromatic contrast, 65 
horizon brightness, 20 
illumination from a point source, 
126 

integrated contrast, 131-134 
optical equilibrium condition, 20 
optical slant range, 30-31 
power requirements, Yehudi cam¬ 
ouflage system, 233 
transmittance, 21 

visibility along slant paths, 29-31 

Glare, effect on color contrast, 72 
Glombs, Yehudi camouflage for, 15, 
241 

Gloss characteristics of natural ter¬ 
rain, 213-215 


Glossy black camouflage paint, 242- 
250 

Goniophotometry, 5, 214-215 
Gray scale for reflectance compari¬ 
son, 209 

Ground glass plate effect, nonexis¬ 
tent, 26 

Ground haze, effect on meteorological 
range, 138, 144 

Handbook of visibility, proposed, 135 
Haze, atmospheric 
effect on meteorological range, 138- 
194 

effect on visibility, 19-32 
value in camouflage engineering, 220 
Haze boxes, 6 

Horizon brightness formulas, 20, 31 
Horizon-scanning photometer, 27 
Horizontal path, visibility along, 19- 
28 

Illumination arrangements 
color contrast studies, 66 
liminal contrast studies, 35 
Image stabilization with rotating 
glass block, 196 

Indicators for liminal contrast studies, 
43-45 

Infrared reflectometer, 4 
Infrared spectrophotometer, 4 
Infrared-bright green paints, 4 
Inherent contrast, 21-22, 74 
Inherent contrast, effective, 130-131 
Inherent integrated contrast, 130-135 
Instruments, field 
contrast photometer, 134 
photoelectric transmissometer, 218 
spectrogeograph, 5, 194-213 
Instruments, laboratory 
automatic recording goniophotom- 
eter, 5, 214 
haze boxes, 6 

horizon-scanning photometer, 27-28 
infrared reflectometer, 4 
infrared spectrophotometer, 4 
Luckiesh-Taylor brightness meter, 

131 

Macbeth illuminometer, 25, 50 
Maxwellian view type photometer, 

131 

photocell photometer, 50 
photoelectric densitometer, 205-207 
photoelectric transmissometer, 21 
Integrating contrast photometer, 131— 
134 

Interchemical Corporation Research 
Laboratory, 4, 27, 242 

Landolt ring test patterns for color 
contrast studies, 61, 62, 69 


LBE-1 Glomb, Yehudi camouflage 
for, 15, 241 

LBT-1 Glomb, Yehudi camouflage 
for, 241 

Lighting arrangements 
color contrast studies, 66 
liminal contrast studies, 36 
Liminal brightness contrast 
effect of shape, 58-59 
in relation to sighting range, 94 
nomographic charts, 78 
Liminal brightness contrast, circular 
targets 

apparatus, 35-45, 50-52 
bright targets, 56-57 
computation of contrast, 53 
dark targets, 57 

effect of visual angle subtended by 
barely visible circular object, 
58, 59, 251-255 

eight-position observation method, 
33 

low-level brightness measurements, 
51-52 

nomographic charts, 78 
observers, 45-49 
photometric procedure, 50-53 
single-position observation meth¬ 
ods, 34, 57 

Liminal target distance 
computation, 74-76 
effect of atmospheric haze, 218 
effect of color, 131 
in relation to sighting probability, 
94 

naval targets in clear and foggy 
weather, 130 

nomographic visibility charts, 76- 
131 

non-uniform targets, 128-131 
Lookout procedure, 73 
Low-level brightness measurements 
photometric method, 51 
psychometric method, 52 
Luminous density in sunlight, varia¬ 
tion with altitude, 29 

Macbeth illuminometer, 25, 50 
Mallo black, 243 

Matte black camouflage paint, 242 
Maxwellian view photometer. 131 
Meteorological range 
defined, 26-27 

effect of atmospheric stratification, 
138-194 

effect on liminal target distance, 
74-76 

measurement, 218 

nomographic charts, liminal target 
distance, 76-131 
Microdensitometer, 23, 205-207 


CONFIDENTIAL 



INDEX 


271 


Model studies 

average reflectance measurements, 

134 

Black Widow finish, 242 
integrated contrast measurements, 
131-134 

ship camouflage, 13 
Yehudi camouflage system, 227-230 
Munsell colored papers for color con¬ 
trast studies, 67, 69 

National Bureau of Standards trans- 
missometer, 21, 218 
Natural terrain 
color of ocean shoals, 8 
color transients in desert terrain, 
8-9 

gloss characteristics, 213-214 
nets for simulating rough texture, 
213 

simulated by matte surface paint, 

10 

spectral reflectance measurements, 
194-216 

texture studies, 213-215 
Naval targets, visibility 
circular targets, visibility charts, 
78-94 

effect of color contrast, 131 
in clear weather, 130 
in foggy weather, 130 
rectangular targets, visibility charts, 
94-125 

under operational conditions, 137 
Nets for simulating natural terrain, 
213 

Night fighters camouflaged with Black 
Widow finish, 243 
Nomographic visibility charts 
apparent brightness contrast, 78 
application to typical camouflage 
problem, 218 

backgrounds other than horizon 
sky, 126 

combined color and brightness con¬ 
trast, 221 

correction for magnifying power of 
binoculars, 128 

correction for uncertain adaptation, 
126 

for aerial use, 144-194 
for signal lights, 128 
liminal contrast, 78 
liminal target distance, 76-131 
naval circular targets, 78-94 
naval rectangular targets, 94-125 
optical slant range, 144-194 
projected target area, 144 
visibility from aircraft, 138-193 
Observation room 
color contrast studies, 66 


liminal contrast studies, 35-36 
Ocean shoals, color, 8, 210 
Optical equilibrium 
along a slant path, 31 
along horizontal paths, 20 
extent, 20 
formula, 20 

Optical slant range, 30, 138, 217 
diagrams, 138-144 
formula, 30 

Orientation spots, liminal contrast 
studies, 42-43 

Ostwald colored papers for color con¬ 
trast studies, 60 

P-61 night fighter camouflaged with 
Black Widow finish, 243 
Paints, camouflage 
see Camouflage paints 
Passive Defense Project, 4-8 
PBM flying boat, Yehudi camouflage, 
239 

Peacetime applications, camouflage 
engineering, 221 

Perception without awareness, 72- 
73 

Perceptual capacity of human ob¬ 
servers 

liminal contrast studies, 33-73 
liminal target distance studies, 74- 

131 

Photoelectric densitometer, 205-207 
Photoelectric transmissometer, 21, 218 
Photometers 

a-c photoelectric telephotometer, 
26 

determination of resolving power 
of the eye, 235 

horizon scanning photometer, 27- 
28 

integrating contrast photometer, 
131-134 

liminal contrast studies, 50-53 
Maxwellian view photometer, 131 
photocell photometer, 50 
Photovolt electronic photometer, 
25 

Physiological factors, visibility of 
targets, 33-73 
Point source 

illumination formula, 126 
maximum angular size, 128 
Projected target area 
effect of atmospheric stratification, 

144 

nomographic charts, 194 
Projected target area, effective, 217 
Projection equipment, liminal con¬ 
trast studies, 36-42 
Psychometric procedure, color con¬ 
trast studies, 65 


Psychometric procedure, liminal con¬ 
trast studies, 53-56 
data analysis, 55-56 
low-level brightness measurement, 
52 

target presentation, 54-55 
test pattern presentation, 67—71 
visibility criterion, 53-54 
Purkinje luminosity curve shift, 61, 66 

Range, daylight visual 
defined, 26-27 

directional variations, formulas, 27- 
28 

in terms of meteorological range, 
218 

Range, meteorological 
defined, 26-27 

effect of atmospheric scattering, 
138-194 

effect on liminal target distance, 
74-76 

measurement, 218 

nomographic charts, liminal target 
distance, 76-131 

Range, optical slant, 30, 138, 217 
Range of target 
see Liminal target distance 
Recognition threshold of colored 
lights, 13 

Recommendations for future research 
luminosity, 29 

visibility of naval targets in foggy 
weather, 130 
Recording apparatus 
color contrast studies, 70 
goniophotometer, 5 
liminal contrast studies, 43-47 
photoelectric photometer, 131-134 
Reflectance, dependence on angle, 
214-215 

Reflectance of camouflage paints 
Black Widow finish, 243 
coffin paint, 242 

Reflectance of natural terrains, 8 
gray scale for reflectance compari¬ 
son, 209 

measurements, 194-216 
underwater terrains, 210 
Reflectometer, infrared, 4 
Resolving power of the eye, 235-236 
Rhoco black, 243 

Scattering, atmospheric, 6-8 
attenuation constant, 20 
Screening of targets, atmospheric, 
19-32 

Shadows simulated with black paint, 

11 

Shape of target 
circular targets, 78-94 


CONFIDENTIAL 



272 


INDEX 


effect on liminal brightness con¬ 
trast, 58-59 

rectangular targets, 94-126 
Ship camouflage, 13-14 
Sighting range, effect of liminal target 
distance, 94 
Signal lights 

nomographic visibility charts, 128 
point source illumination formula, 
126 

Sky-ground ratio, 217 
formula, 32 

Slant downward path, visibility along, 
29-32 

Slant range, optical, 30, 217 
Spectral reflectivity of natural ter¬ 
rains, 8, 194-210 
Spectrogeograph, 5, 194-213 
adapted for goniophotometric meas¬ 
urements, 214 
calibration and use, 204 
identification of target, 197 
identification photographs, 200 
image stabilization, 197-198 
installation, 203 

oblique terrestrial mirror system, 
203 

optical system, 198 
periscope and sky mirror system, 
202 

sight mechanism, 206 
spectrograph, 198 
Spectrophotometer, infrared, 4 
Spectrophotometer for aerial use 
see Spectrogeograph 
Stabilization of image with rotating 
glass block, 197 
Standard atmosphere 
defined, 29 

density in molecules per unit vol¬ 
ume, 30-31 

Standard lamps for liminal contrast 
studies, 50 

Standard test surface, liminal con¬ 
trast studies, 50-51 

Stiles-Crawford effect, photometer ac¬ 
curacy, 131 

Stratification of the atmosphere 
effect on meteorological range, 138- 
194 

optical standard atmosphere, defi¬ 
nition, 30 

Strip-camera, disadvantages for aerial 
photometry, 196 


Subtense angle 

effect of brightness contrast, 59 
formula, 74 

Target presentation, liminal contrast 
studies, 54-55 
Target visibility 
see Visibility of targets 
Target visibility charts, circular tar¬ 
gets, 78-94 

Target visibility charts, rectangular 
targets, 94-125 

TBF torpedo bomber, Yehudi camou¬ 
flage, 14, 239 
Telephotometers, 25 
Temperature-sensitive pigments, 10 
Threshold confidence in sighting tar¬ 
gets, 94 

Tiffany Foundation 
camouflage of sea search aircraft, 
225-241 

laboratory for camouflage field 
studies, 12-13 

screening of targets by atmosphere, 
22-26 

visibility of targets, 33-73 
Tone-down camouflage measure, 220- 
221 

Transmission of light through atmos¬ 
phere 

atmospheric attenuation constant, 
20 

experimental studies, 23 
transmittance formulas, 20-21 
transmissometer, photoelectric, 21, 

218 

Underwater terrains 
color of ocean shoals, 8 
color variation with water-depth, 

210 

reflectance studies, 210 

Visibility criterion, liminal contrast 
studies, 53-54 
Visibility of targets, 19-32 
along a horizontal path, 19-28 
along a slant downward path, 29-32 
apparent and inherent brightness 
contrast, formulas, 21-22 
apparent brightness formulas, 31 
atmospheric attenuation constant, 
20 

black targets, 22-26 


computation of liminal target dis¬ 
tance, 74-131 

daylight visual range, 27-28 
dependence on contrast, 71-72 
effect of color, 131 
effect of haze, 19-32, 218 
effect of non-uniformity of target, 
128 

effect of shape, 94-126 
experimental studies, 22-26 
from aircraft, 138-193 
influence of color contrast, 59-72 
naval circular targets, 78-94 
naval rectangular targets, 94-125 
naval targets in clear and foggy 
weather, 130 
of signal lights, 126 
nomographic visibility charts, 76- 
131 

optical slant range formula, 30-31 
physiological factors, 33-73 
white targets, 22-26 
Visual acuity, dependence on con¬ 
trast, 71-72 

Wake camouflage, 13-14 
Water surface camouflage, 9 
White targets, visibility studies, 22-26 
Wiblack, 243 

Yehudi camouflage system, 14-15, 
225-241 

alignment of lamps, 237 
color correction, 234 
demonstration of Yehudi principle, 
225-227, 230-233 
effect of cross winds, 238 
effect of paint color on power re¬ 
quirements, 238 

for B-24 Liberator bomber, 14, 227- 
230 

for LBE-1 Glomb, 15, 241 
for LBT-1 Glomb, 241 
for PBM flying boat, 239 
for TBF torpedo bomber, 239 
homing course for Yehudi equipped 
planes, 238 

lamp spacing, 229, 232, 233 
model studies, 227-230 
photoelectric intensity control, 237- 
238 

power consumption, 225 
practical power requirements, 233 
theoretical power requirements, 233 


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